[zaroth]

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.

[Khoth]

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.