The idea is that nobody is going to get a million dollars in revenue from a game without being visible enough that Epic's receivables department can bill you. So they can ignore the 99.99% who download it and never get a hit.

I'm amazed that 400,000 people have downloaded Unreal Engine, though. It's really complicated, hard to use, takes hours to compile, and is only worth the trouble if you're making an elaborate 3D game. Not that many people do that.