Apps don't make anything near what an actual purchase does. Plus, nobody looks at ads when he plays a game...
1. He often made more money from the ad-supported free versions of his games rather than the paid versions.
2. The Play Store is not the only place to download/pay for Android games. According to him, there are hundreds of alternative markets (such as Amazon's), some of which are much more prevalent outside the U.S.
Therefore any measurements should count revenues based on ads too, at least on Android where the freemium model is often preferred.
Also, point #2 is very valid. Besides alternative app stores, developers on Android can also self-publish on their own website and experiment with various kinds of payment. See for instance The Humble Bundle, which allows people to pay how much they want (much like donations) and which had a nice return until now ...
Besides being just one account, that might as well be because, well, his paid versions didn't sell that well at all?
[1]http://techcrunch.com/2010/12/03/angry-birds-android-1-milli...