yaix2 - 8:56 am on Sep 16, 2011 (gmt 0)
Just a guess: Advertisers set maximum click cost and daily budget in AdWords. Campaigns with a high max click cost are displayed first and more to the top, because of the real time auction in Adsense. So they get clicked first; but their daily budget will be exhausted quicker for the same reason. So towards the end of the day, only the lower paying ads are left and get displayed.