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 less room to do so. Google has a cash cow and they've only really been working one or two of the squeezable portions, somewhat halfheartedly.
3) Google is cannibalizing the organic results for high money verticals, for example with consumer finance comparison products. Why have Citibank pay an affiliate $200 to buy $160 of AdSense ads to pay Google ~70 when Citi is perfectly happy to pay Google $200 directly?
4) Routine optimization works. Google+ gets all the ink, but Google is keenly aware that they have two products: core search and AdWords/AdSense. If you thought Circles got some programming chops thrown at it, pretend a 1% improvement was worth billions and you had infinite R&D resources.
5) Something else entirely.