Those extra pages in a book may be index; bibliography; afterwards; a sample chapter of the sequel; a sample chapter of an unrelated book; a collection of short stories or essays related to the book; advertisements for other books; forms to fill out to request catalogs from publishers. There are all sorts of reasons for having non-main-sections at the end of a book. I'm more likely to expect them in non-fiction, but not always as much as there is.
And it still usually trips me up. I was expecting there to be more story/plot/argument.