Same thing. My game was halfway to top 100, and then it got Greenlit out of the blue. Well, not really - I have added my game to a bundle a few days before that, and published a demo version. So I think Valve looks at the game's activity and how much the developers actually care. Plus, they might have liked the game generally.
The problem I see is too many games getting into Steam nowadays so it's becoming more like Kongregate - people don't have any guarantee their games will do well or even be noticed. I guess that Greenlight was once proof of a game's good quality, but now devs have to assure their games are good.