Grade system is bugged afaik - the "grades" are just divisions based on certain rating point ranges.
It's supposed to be an ELO-type rating system, with your rating changing depending on the results of each game. Factors include your rating, your partner's rating, and the ratings of your opponents. Wins give points and losses deduct points.
I know there is a detailed explanation in a thread somewhere, but I couldn't find it. Some good examples are given there though - basically losing a scrim or winning a pub wouldn't change your rating much, but winning a scrim or losing a pub would.
Another example is that if you played 4 games, each with people who are all the same rating, then winning 1 and losing 3 of those games would result in a net 0 rating change.
Anyway, seems like sometimes large chunks of points are given or taken away from some people at random/based on some bug, so it's largely irrelevant :P
Edit: found the thread:
viewtopic.php?f=37&t=63