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2. Google has something like "referer value". Visitors from digg is not visitor from Google or related PR9 site.
3. Google has something like "geolocation visitor value". If you're coming from "bed converting" region (google analytics data), your click will be underpriced, even if advertiser isn't limit campaign on particular region(s).
4. Google track your clicks across google search, google ad network and google analytics and try to predict your buying potential and most efective CPC.
This is not confirmed in any way - but I believe adsense logic has this and many more features.
Sorry for my english, russia speak. :-)