What does the new Coverage % column mean in Google AdSense reports?

I'm not positive but I thought it meant the amount of times in percentage that the search engine can actually show ads that are relevant to the content of the web page. Now, if this is true then there is a little problem because I've notice my ads change according to searches performed from my IP address; the search engine now seem to factor in my my search history to figure out the best ads to display that would cause for a click through. Therefore, I probably am wrong about coverage and as there is little if anything substantial in Google help about this, I'll shut up now.