Google put out a new Webmaster Help video responding to a user-submitted question about Google's estimated numbers for results on queries. The specific question was:
How reliable is the site: query and why does the total count sometimes change from page 1 to page 2?
"I wouldn't really put it in terms of the "site:' query," responds Google's Matt Cutts. "Instead, I would pivot, and narrow, and focus down on the results estimates. So there's a few things to note. If you haven't noticed, you should notice that results estimates are only accurate to about three significant digits. So, they are estimates. As we're looking for queries, at some point, we say we have enough to constitute the first page, and we see how far we've gone through our posting list - the amount of data we have. Therefore, we can estimate, there's about this many results left."
"Now, when you click from page one to page two, suddenly you're going deeper so you have a better estimate or you have a slightly different estimate because you're starting to look not just a little bit of the way through all those documents," Cutts continues. "You're actually starting to go deeper and deeper. So, it's relatively common for our results estimates to change, going from our page one to page two, typically becoming more accurate. So it's not usually a factor of the 'site:' query. It's usually a factor us digging deeper through our data, and as a result, we have a better, or at least a different estimate for how many results there are."