They didn't all have premium versions to pirate? Angry Birds didn't.
Some of the top games probably were noticed randomly by the right bloggers; a Russian blogger loved my game, and it's sitting in a nice comfortable position in the Russian Market ranking, for example. Angry Birds was probably featured in EVERY blog that deals with Android games.
Some of the top games also have annoying but effective viral marketing components; I've so far avoided stooping to those depths.
Some of them probably had better marketing plans. I'm a programmer, not a marketing expert.
And if someone inside Google decides randomly to feature a game, then it's set. My game WAS featured on AppsLib, a secondary market, and after it was no longer featured it stayed on top of the free apps ranking at #2 for weeks. As in, the #2 downloaded APP, not game.
Unless you know more than anyone else outside of Google about the actual algorithm Google uses to determine how apps rank, you have no clue as to what could have caused a game to fall -- but a huge fall happens the SAME DAY it was pirated.
It's not proof, but it's a very suggestive correlation. Just because it doesn't match your world view doesn't make it garbage. What's indisputable is that the piracy DIDN'T give me a boost in sales OR downloads, which completely invalidates the "piracy is just another kind of marketing" theory.
There's a lot of random luck involved in getting to the top organically, and the process is potentially fragile unless you've got a lot of cross-promotion you can apply from other apps. Or a lot of money to buy ads. I had neither.