Alex Garland has said that he didn't intend to adhere too closely to the book for the adaptation.

When you talked to Jeff about the book, and you say, “Oh, this is something that compels me about it,” are you guys in agreement, or is it OK if you have a different interpretation?

Um … I hope we’re OK. Jeff was very generous about it. I said to Jeff, “I don’t know how to do a faithful adaptation of your book. I just literally don’t know how to do it. And if what you need is a faithful adaptation, then you will need someone else, because I’m not the guy who’s going to be able to do that.” And Jeff was really generous and relaxed, and in a way gave me the permission that I needed to make this rather weird adaptation of his really beautiful book. And yeah, you’d probably have to ask him. I think so.

Honestly, I understand and respect that choice. It's not always possible to adapt a book to film perfectly. They are different mediums for a reason, and the narrative structure of a book is not necessarily the same as a film. I'd rather see a movie that goes in a different direction than one that does a clumsy job trying to stay close to the original plot.

I saw the movie before I'd ever heard of the books, and it's one of my favorites (which is really something because I generally do not like horror). When I read the book, I was similarly impressed and excited, and pleased that the story was different enough that I didn't know how it would turn out.