This is coming from Steam themselves:
The specific number of votes doesn't matter as much as relative interest in a game compared with other games in Steam Greenlight—we need customers to help us prioritize which games they want to see made available on Steam.
We're going to be reaching out to developers as we see their games getting traction regardless of whether they have achieved a specific number of votes or are sitting 1st or 2nd place at any given time. We are most interested in finding the games that people want, not requiring them to hit a specific number of votes.
Basically, votes are used to see how popular your game is in the community. Then Valve looks at the most popular games and considers putting them on the store.