The iOS publishing plan is simply the easiest way for a developer to get their games on a widely adopted platform that has a built in means of generating and collecting revenue.
That's really all there is to it in explaining the numbers.
If you're an aspiring game developer and don't have the resources in man power, funding or development tools for even a modest budget game on a closed platform, developing and publishing through iOS means very little risk/exposure even if success stories are certainly the exception rather than the norm.
Just about any programmer or game developer could potentially come up with the next great idea and have it developed into a commercial product that generates revenue.
Naturally, it helps that the majority of iOS games have a rapid turnaround time with minimal investment and are often the price of a music track on iTunes, but not all great game ideas require 8 figure budgets.
With the iOS, the big issue for the developer is simply marketing as there is little odds of being discovered accidentally without significant word of mouth to draw attention and sales.
Android should be generating similar numbers given enough time as it is based upon the same ease of development type system.
