In order to play only 1 game at a time, you need at least 30 unexpired games
Also, you lose/gain points if the opponents you have played, in finished games, do better. for example if we play, and I beat you, and then drop 700 points in rating, then your rating will drop as well.
The Real-Time Ladder uses Microsoft's TrueSkill which just gives you results based on each game instead of recalculating the whole system each time. The 1v1 and 2v2 ladders use Remi Coulon's Bayeselo which recalculates everyone's rating/ranking each time it runs.