As with all adaptations of books, there are decisions that have to be made about characters and plots. What's interesting about all these version is that none of them end the same as far as I recall. In fact although they follow the basic elements of the original story, they all differ in the detail.
So what's important? Is it more important to be as faithful to the detail of the original story as you can, or is it more important to be faithful to the central theme? If you took someone who's read the book and then someone who had seen the first film, a third person the second film etc., and asked them about the plot, you would get a pretty similar outline. Essentially our hero encounters a spy who gets murdered and Hannay falls under suspicion. He evades capture by both the police and the enemy spies and uncovers a plot which he eventually foils (in the films at least) by solving the riddle of the 39 steps.
This is, roughly speaking, how oral tradition works. It's the key elements that are of significance, and they don't change. Settings might vary, some characters might change, but overall the story follows a set pattern. To our still modernistic ways of thinking, variation is the enemy of accuracy, but it doesn't have to be seen that way. Variation can authenticate the core message.
So it is with some of our biblical records. They differ, but the core message remains the same.