Some people read secondary sources or worse. Others read primary sources. And others read both.
Some people read one book on a topic and assume it is true. Others read ten and compare their stories.
Some people have high standards for what they will accept as scientific evidence, and others low. Some people read newspaper articles about studies. Some read the actual studies. Some just read the conclusions and say science has supported those conclusions. Others evaluate whether a coherent explanation has been presented, and whether the evidence is sufficient for the conclusion.
Some people look for authorities to get knowledge from. Others use their own judgment.
People who could be taken as authorities themselves have varying standards that they subject their own knowledge, and publications, to.
Some people are easily fooled (not that anyone is doing it on purpose). Some make it their mission in life to find truths and avoid being fooled.
Some people think mistakes and rare and don't worry about them very much. Others think mistakes and common and are always vigilant against them.
Some people select, say, a Burke biography based on the dust jackets. Others read 2 chapters from each candidate book to judge the quality for themselves, then read the one with the best author. And some other people insist on reading all the books, even the bad ones, in case they contain a useful tidbit or an astute argument.
Suppose Bob has the highest standards in every regard mentioned and many others, with regard to some topic he likes, say Xenophanes. All his knowledge has about Xenophanes gone through extensive criticism and vetting. He has never assumed that some author correctly read the primary sources; he always reads them himself to check. When he reads a statement saying, "On the issue of X, the best analysis was done by Sarah Parker. And she concluded Y." he is suspicious. That statement does not contain any arguments he can judge. He'll have to read Parker's book or disregard it. Bob is always wondering how an author knows what he claims to know, and whether anyone involved could have made a mistake (the author, a translator, Xenophanes' contemporaries who provided commentary and quotes about him, the editor who must have made at least a few edits the author wasn't pleased with, etc). Bob always compares quotes with the author's statement of what the quote means and decides if he thinks that's accurate. If he's not sure, he won't take the author's word for it. If the author doesn't give enough quotes to allow a comparison so Bob can judge if the author is interpreting the material correctly, then Bob becomes suspicious and guarded.
One day Bob meets Joe and they get into a discussion about Xenophanes. Joe is different than Bob. He only reads primary sources when they are quoted in secondary sources. When an author unequivocally states that X is true, Joe believes X to be true. When an author cites an authority as saying Y is true, Joe figures if it was good enough for the author it's good enough for him too. Joe knows he's not a world class expert. His knowledge is pretty good, and pretty reliable, but better is possible. But better isn't needed; it's good enough; it's reliable; it's accurate; it's genuine knowledge gleaned from serious study. He's looked into things in as much depth as he could, given the other things in his life, and it's a lot more than most people do. Joe knows not to trust everything he reads on the internet, but books are different and more reliable, especially the ones by PhDs. They do some of the work for Joe, and Joe can reasonably accept the help in his own truth seeking. That's what Joe believes.
Bob and Joe debate some issues about Xenophanes. Bob says what he knows on some subject, and Joe says what he knows. It contradicts. Both give their summaries of their recollection of some supporting evidence. Neither convinces the other. Both find the other says things they are quite sure are false based on their existing knowledge.
Bob sees what's going on. He remembers some of the books Joe refers to, and the flaws in them. He tries to say that he's read those books, as well as others. Joe says, "Don't ask me to accept your story on authority. Just because you read more books doesn't mean you're right." Bob pulls out a laptop and finds excerpts from one of the books. He shows Joe some internal contradictions, and then finds another source with a better take on the matter, shows Joe, and notes it does not have any internal contradictions that he can see. Joe says, "Look, I've read several books, and they back up this one. Maybe it made a mistake in this one area. Maybe you remember all the details better than me. That doesn't mean I'm wrong. It's not like you are a world class expert; these authors know more than you, and they sometimes contradict each other. I'm not just going to take your word for stuff. And you say this new source you offer doesn't have mistakes like mine does, but I haven't read it and checked it, and I'll bet I could find some criticisms of it if I googled around."
Bob starts to despair. Joe has no knowledge of sufficient quality for Bob to find it useful; Joe has nothing to offer. Joe doesn't have the appropriate attitudes to create knowledge of the same quality Bob is interested in discussing. And anyway, at low precision, who's to say which side is correct? If you simply ignore all the difficult details, and ignore whether theories can survive the harshest criticisms possible, you can make a perfectly good case for many different views. If you don't ask whether explanations solve the most hardest problems around, and accept ones that simply solve some subset of the easier problems, then you lose the power to differentiate between very good ideas and mediocre ones.
Bob has an idea. He tells Joe about standards of truth seeking and quality of knowledge. He tries to explain that Joe's method of approaching the subject isn't rigorous enough to find the truth. Joe says, "You're holding knowledge up to an impossible standard. No one lives up to this idealized version of truth seeking you speak of. You're giving a generic argument for rejecting the vast majority of existing human knowledge. You're finding tiny faults and then trashing whole enterprises."
Bob says, "It's not an impossible standard. I live up to it. And so do some of the authors I've read. Karl Popper's analysis of Xenophanes, for example, lives up to this standard. I can give you a list of the best authors if you'd like."
Joe says, "I've read about that Popper fellow. High standards? Hardly. His Plato scholarship is very questionable. Maybe he has his merits, but he's no perfect angel. And as to you, I'll overlook your arrogance, but I won't accept that you're some super genius authority without a bit more proof."
Bob says, "What sort of proof would you accept?"
Joe thinks for a while and says, "I guess I'll know it when I see it. Persuade me."
Bob says, "Yes, but how?"
Joe says, "Well, you're the genius. You tell me."
Bob says, "I've told you. You need to consider my statements, and everyone else's, according to this higher more rigorous standard. If you do that, you'll see that mine live up to it. Many of the things you think you know do not live up to it. Please drop your bias in favor of your existing knowledge and open your mind and try to learn. It's hard, but by an effort we can make progress."
Joe says, "Putting down all my existing efforts to learn things isn't friendly or persuasive. And I'm not going to make this huge effort to do as you ask before you've convinced me it's worth doing."
Bob says, "How can I convince you it's worth doing when your standard of judging ideas is wrong, so you can't tell what is worthwhile or not?"
Joe says, "Oh come on. I may not be perfect, but I'm not that bad."
Bob says, "It's not up to you to decide what is too bad, and what is an acceptable level of badness that won't hurt anything. It's not a human choice what is and isn't required to find the truth and correct errors."
Joe says, "Yes, I understand that. But my standards are pretty high. That aren't low. I accept truth finding is hard, and I make a serious effort."
Bob says, "Your standards are low enough you won't listen."
Joe says, "Disagreeing isn't failure to listen. I've been insulted for the last time. Goodbye."