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OK, I have a completely sound explanation for all this madness. Google has a large hopper full of ping-pong balls with a different Adsense publisher id on each one. Every day they draw five balls and those accounts get to do well. Everyone else gets to wait for the next drawing.
Mentat: The big $$$ is on search, not on display, that's why you see so many programs trying to install adware (toolbars) with custom search engine.
The truth is that search ads are much better paid and have much more effect