Yeah, I'm fairly certain that when people disconnect they get a loss for each person they are playing, and the people who didn't drop each get a win off of the disconnecter.
i.e. if you are racing four player, at the end of the four match set you get a win for each person you are ranked ahead of and a lose for each person you are ranked behind. If you disconnect, you are automatically ranked at the bottom, meaning you get a loss from everyone playing, and each of them gain a win off of you. This makes disconnecting a *really* bad idea if you are playing for a score. Even if you can't make first, it is worth it to try to get second or third, as you will still add wins and prevent losses. Disconnecting from a four-man race is an automatic 3 losses, which is painful.
As long as you are playing with more than one person I think you always have a chance of gaining a place on the last race, so really you should never disconnect. Plus, if you disconnect just cause you can't win for the last race, it takes longer to reset the game and connect online again than it does to just finish the race. In short, disconnecting is pretty dumb