Craig writes: "Thanks for the great question. We get this from time to time and hopefully I can clear up some of the confusion. The number of estimated pages listed to the top right of a Google search results page is indeed, an estimate. It's a good estimate but still, an estimate.
There are many reasons why one might see a difference in the estimated number of pages returned for the same query. It's most likely the queries made by your co-workers were sent to different Google datacenters in what appears to have been a round-robin fashion. The index at any given Google datacenter can change slightly over the course of a day (each index is refreshed completely every three to four weeks). Depending on which datacenter finishes a query, the estimated number of results may vary.
Without having direct access to your environment it is hard for me to tell for sure, however, I believe this is the case."