When you perform a search, the results are often displayed with the information: About XXXX results (X seconds).
Google's calculation of the total number of search results is an estimate. We understand that a ballpark figure is valuable, and by providing an estimate rather than an exact account, we can return quality search results faster.
In addition, when you click on the next page of search results, the total number of search results can change. Google's search index is constantly changing, and your second search results page may come from a slightly different version of the index than the first page.
If you're a webmaster and you've created and submitted a Sitemap, you can use the Sitemap details page to see how many of your listed URLs have been crawled and indexed by Google.