Why Do Google Hit Numbers Vary?

Supa-Fly writes "I have a question about some conflicting results with the search engine google. I did a search for "pictures of mountains" and got exactly 1 million results. My friend did the same search (from the same office)and got 1,010,000 results. A second friend did the same search as the last 2 and got 1,020,000. These have not changed and every person gets the same results each time. My question is what is up with the discrepancies on google's search results?" Since this question is hard to answer from the outside, Craig Silverstein of Google kindly supplies his best answer to this question, below.