These day they just estimate the size of the resultset. It's pretty easy to come up with queries where it estimates the wrong row count until you paginate to a later page. I think the theory is just that approximately nobody clicks on more than the third page.

Example on first try: https://www.google.com/search?q=foobarfrak navigate to the last result. And you get a really misleading "No results containing all your search terms were found." page...