Also, over time, the book wears out, so someone makes a copy. Mistakes and misunderstandings creep in. Or, even, the copyist changes a few things to better fit

Also, over time, the book wears out, so someone makes a copy. Mistakes and misunderstandings creep in. Or, even, the copyist changes a few things to better fit his world view. It is possible that the book isn't as static as you think.