Thanks, BOAC, for the info. So what happens when try to click on page 32? Do you get an error message? Do you simply stay on page 31? Or do you go to a page 32 that displays nothing?
It's possible that post editing may be related. If a mod, say, deletes five posts quickly, that would reduce the number of total posts and thus potentially affect total pagination. The most visible context for this is, as you observed, posts at the top and bottom of pages. But unless heavy editing is going on, that wouldn't explain page count overages of, say, two or more.