Returning accurate search result counts (i.e. "give me an exhaustive list of all pages that match this query") is at cross purposes to the main search problem (

Returning accurate search result counts (i.e. "give me an exhaustive list of all pages that match this query") is at cross purposes to the main search problem (i.e. "give me the best ten pages for this query"). For the former, you want to be exhaustive, for the latter, you want to cut every corner you can to get results back as quickly as possible. Google optimizes for the latter at the expense of the former. Basically, don't put too much stock in any of the numbers you get back from Google.