hi Mr. P

the search results are already diluted by your language, your location (all the way down to your city), other environment information of your browser and by datacenter of google itself which holds diffent aged datas in each datacenter. It might also be fluctuating by the common trends (see google trends) for that given "season" for your keyword (a search for "presents" gives relevant sites for "christmas presents" - I'll set a reminder to check for "presents" on valentines day).

Google will most likely not populate your top 10 spots with your top 10 visited sites for a given keyword as that would not make sense for finding new exciting stuff for you which you might like more and which would be more relevant... Other users will have a different order of results... I bet THAT makes a difference, though.

DEV