It's important to note that not all revenue is tracked by Apple (and hence realized in Top Grossing), much as they've been trying to take 30% of everything. Man

It's important to note that not all revenue is tracked by Apple (and hence realized in Top Grossing), much as they've been trying to take 30% of everything. Many free (and even paid!) apps make a very, very large amount of money off of both third-party in-app advertising and revenue/placement tie-in offers (i.e. "put my product in your game, make $$$"). So while this article is a good plug for A Thinking Ape it's important to note that "freemium via IAP" is not the only "freemium" iOS app model and certainly not the only way to generate revenue, and hence the Top Grossing list is not an accurate summary of the actual top grossing apps or developers across any given year.