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Maybe Google doesn't pay as much for click happy visitors? Surely they know the IP address of the clicker.
People who have already clicked on an ad are much less willing to keep clicking on more ads.
I take myself for an example. If I search for something and end up clicking on an ad instead of a search engine result listing I expect to find exactly what I am looking for at the website I get to from clicking the ad NOT more ads so I never click ads in this case just leave if the site does not have what I want.
Maybe your AdWords and Overture visitors are more like me and will not click on more ads?
I've noticed no significant change in EPC during this switch over to Adwords visitors. And my CTR is up a bit, so it appears once a clicker always a clicker.