Matt Cutts made a comment in this thread over at Threadwatch.org in relation to the problem with getting accurate data using the site: command in Google:

Anyway, I believe I’ve said that 72.14.207.104 is the most likely to return accurate site estimates because it has some new infrastructure, and that I hoped the new infrastructure would be at all data centers by the end of summer, which would be about another month. Bear in mind that we sometimes take data centers out of the rotation for various reasons, so even a single IP address 72.14.207.104 can hit different data centers at varying times. And different data centers will show different results estimates, which accounts for why you see different things when you try different IP addresses”