That's because there are some deleted posts in the thread.
Our forum software paginates everything to 35 (I think) posts per page. When a thread gets that 36th post, it starts the second page, and so on. When we (the moderators, that is) delete posts, they don't actually go away, it just doesn't show them to you guys any more, but they stay in the thread. Thus, if that thread has 40 posts, but 5 are deleted, the thread will say it has 2 pages, but it only displays 35 posts, or one page worth, to you guys.
Confusing, but that's how it works.