Analyzing my web access log files I find that my web pages change positioning in serps almost every day. If yesterday one of my pages could be found on the page #2 today I find it on the page #1 or #3. What is possible reason? Is it daily updates of the index?
While google may not re-assess your site everyday it is constantly evaluating the sites around you as well which is why there is constant re-shuffling. It is also using more than 1 data centre currently which means it is showing more than 1 set of results.