1. Adsense will look at search phrase if you're refered from popular search engine.
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.
I have noticed that the ADLinks on my pages do refelect the key word that I used to search for my own pages on my own site. So Google is indeed looking at the words used to find the page with the AdLinks on it.