In this blog post, Markus says that his Adsense click through rate had dropped 60% in two months as a result of the clickable area changes made to Adsense ads. What that suggests to me at least is that a lot of his clicks were accidental, which is usually due to deceptive ad formating or ad positioning.
Maybe there's something else going on, but there has always seemed to be something strange going on with Markus's claims.
I don't really doubt his revenue claims, although he's been throwing around the same revenue numbers for something like 2 years now... However, I suspect there is a lot more to the story than Markus would like us to believe.