When I worked at Google in 2006/2007, Larry and Sergey held a Q&A session, and this exact question was asked of them. One of them answered (I don't remember which) with the following humorous response (paraphrased):

"When I open up my email, I start at the top and work my way down, and go as far as I feel like. Anything I don't get to will never be read. Some people end up amazed that they get an email response from a founder of Google in just 5 minutes. Others simply get what they expected (no reply)."