Some combination of: 1) Panda, which redirected hundreds of millions in revenue from some content farms to other parties (including, likely, the ether) which do not show up on Google's books 2) Google is being increasingly aggressive about monetizing the Google SERPS. More ads in more places with more prominent billing. Other sites on the Internet have not been aggressively up-monetizing principally because they have…
One more aspect: a lot of PPC advertisers do not like to use network ads (i.e., partner sites) because they are perceived as yielding lower quality clicks. Clicks from Google itself are seen to be more likely to monetize for the advertiser.
For instance, only advertising at certain times of day, or certain keywords for specific regions, not advertising in specific geographical locations, the use of quotes and brackets in keywords, etc...