Clarence - 1:43 am on Dec 29, 2012 (gmt 0)
Advertiser don't have unlimited budgets.
Think like an Adwords' Advertiser. You spend $25 a day. In Adword you can choose "spread money out during the day, or get as many clicks as soon as possible"
Advertisers budget is $25, so once their budget is used up for the day, their ad's stop running, and cheaper advertisers take their spots.
In some niches you have to be the first to get the Traffic "and ad clicks" for the day to take advantage of the Ad budgets for the day before it runs out.
A lot of Adsense Advertisers don't realize this! I doubt most Adsense Advertiser ever played around with Adwords on the content network to see the other side of the coin.
What you are seeing is Daily budget drain for the High end advertisers, then after the budgets are drained, the excess advertisers take over.
Tip: Try to drive as much traffic as possible at the start of the day to capture as much of the Ad budget as you can.