aristotle - 3:24 pm on Dec 26, 2011 (gmt 0)
Google is steadily accumulating more and more data on all websites, especially data on user behavior. For example, if a user bookmarks a site as a favorite, or visits other pages on the same site, these are positive signals. But if a user stays on a site for only a short time, then returns to the SERPs, this is a negative signal.
As Google accumulates more and more data on a site, its rankings may shift up or down depending on the various signals.