[mywittyname]

There's probably more to it than that, and Valve just isn't telling us all the details.

It's likely they are weighting accounts that have a lot of (recent) game activity and game purchases as well. Plus, they have access to hardware information via their hardware survey, etc.

[BlueTemplar]

It's certainly possible that Valve is not being truthful about the lottery. (Would that be illegal for them to do ?)