Also, they have to keep track of all the different versions of history they've read. It's easy for us to spot continuity errors like e.g. Anna Valmont telling the "wrong" story in Skin Game of how she first got involved with Nicodemus, because we only read one version of Death Masks. For the beta readers it's probably harder, and for Jim himself keeping track of what "actually" happened must be a nightmare.
In one of the Q&As around the Brief Cases release Jim said exactly this. I happened to transcribe a Q&A for another group and here is what Jim said:
"Oh, I don’t check stuff. I don’t check stuff because it would be useless for me to check stuff. When you’re writing a book you’ll go through and plan it out and then get the rough draft written and then you’ll clean that up and send that to beta readers. Then you’ll go through and changes things based on feedback from them, go through it again one more time and see if there’s anything that that needs to be done, then it goes off to the editor and the editor comes back with some comments and then you change it some more. Then it goes off to the line editor and then it comes back and you’ve got to fix all the grammar and spelling errors and then once you’re done with that it goes to the proof stage and then you’ve got to read the proof. So by the time the book is done I’ve seen between 9 and 11 slightly different versions of the same story and in my head I can’t keep track of which one wound up in the books. You guys have read one version. I’ve kind of got this cloud of parallel universes rather than a working knowledge of what actually happened in the book universe. So that’s why I have friends. Priscilla Spencer is very good at that. My friend Lowell in Chicago is really good at that. And when I say “really good” what I mean is it is so annoying. But really, really necessary. Because I’ll write this big scene and be like “boom boom boom boom POW! Wow! That was really good!” and I’ll send it off to the beta readers and they’ll come back and be like “Yeah, you killed that character three books ago.” Stuff like that happens to me." - June 7, 2018, Brief Cases Book Tour - Joseph-Beth Bookstore in Lexington, KY
https://youtu.be/Df_3c02lelg