Wasn't, like, Halo3 released when the 360 was released too? And it was released before the PS3?
And the fact that the 360 was cheaper than the PS3...
and the fact that Halo3 was the only half decent game to play online (that is, if you could support the hyperjumps and kids spitting in their mics yelling at how noob you are with the RPG) for quite a while.
And the fact that some people probably bought the same game a few times because of scratched disks.
And what about LANs and stuff?
I guess you get the point now...
It's like if people would come and say "ZOMG STARCRAFT OUTSOLD *insert borderline shady recent game here* HOW SURPRISING"...
I'm sure a company doesn't care if a person buys it 1, 2, or 40 times, a sale, is a sale, I wouldn't be surprised if it was xbox's more pirated game too.