Have you ever wondered why the results for a certain query are always the same? Do you have a site ranking 5th for a major keyword and no matter what you do it just won’t move up to 4th?
We call this phenomenon Search Result Lockdown.
Basically the search results for popular keywords behave almost as if Google is just loading them from a cache (perhaps they are, it would save server load) and the cache is only updated once every so often. Thanks to the QDF algorithm getting to the top for fresh queries is quite easy at the moment but trying to break a well established search phrase can be nearly impossible.
My theory is that there is a certain amount of caching or intentional lockdown of the popular search results so that Google can tweak the algorithm without affecting the “money” phrases too much. They can build click data on the long tail results to test new systems while the main keywords show the normal high quality stable results.
Whatever the reason the end result is that sometimes search results don’t change for weeks and even months.