If we're in the top 100 I'm pretty sure we're considered, though better rank probably means much better chance to get picked.
It's likely a mixture of this and comparing it's growth in votes over time.
It could just be that say, top 20~ get in, but they probably take the rate of votes into consideration too.
If you have a game that is moving on up rapidly and is at #40 and it's been on Greenlight for two weeks, I'd imagine that would seem to be in more demand than something that has been on for 2 months and sat at #15.
Eitherway, I've no doubt that the games that do get chosen are those which have the most votes and seem to be the ones that will make Steam the most money. So, being higher ranked just proves you're that sort of game.