What is Googles content matching scheme? Is it based on what is in their index since they crawled the site? How much does it weigh meta keywords and descriptions? I would imagine for a blog having relevant ads based on conent would mean it would have to index the adsense sites more often since the content changes daily or hourly.
Title of the page, filename, content, etc etc !! Everything counts. The Google Mediabot partners spider the adsense websites separately to server related ads.