BigDave - 10:49 pm on May 5, 2008 (gmt 0)

My completely unfounded guess is that it is related to adwords landing pages with adsense, but there has to be at least one or more additional factors.

For example, what sectors are the sites operating in? A big money sector like travel is subject to a lot of abuse. There is also a lot of competition amongst publishers so Google doesn't really need that many sites to use up their inventory.

For that matter, if it is a page about Sven's Ice Fishing Resort that is buying ads from pages with pretty much the same information, you aren't really helping the user experience, and you are taking slots away from Arctic Al's Ice Fishing Tours, that actually sell you the reservations.

I just think there is more to it than having an adwords account.