Is it all threads or can you point to a specific example? If citing an example, it might also help saying exactly when you tested it.
One glitch I've run into is where the thread has exactly the right number of posts to fill a page (whatever your posts per page settings might be.) The server software then reports that there is one more page than actually exists, and the extra one points back at the real last one. Try comparing the reported number of posts in the thread and see if it corresponds to the number of pages it's claiming exist. (I think I've seen this on other sites running vBulletin. I have a faint memory that it doesn't always happen either, but may only show itself in threads where the mods have deleted posts or moved them somewhere else.)