Google cheats, but in a way that very few users will notice. You can only access the first 20 pages of search results. Depending on user behavior, this is one way to offer navigation via page number while limiting worst-case cost.
I doubt that 21st page actually exists, even on the back end.
I would bet the top-line number of results is some sort of extrapolated estimation thing, a hand-wavey way to represent how deep the hypothetical result set is for that query.
It definitely is an estimate. If you search for a rare enough term that there aren't 20 pages of results, it can still estimate thousands of results until you call its bluff by going to a later page, at which point the estimate is replaced by the actual number.
