There are handicap leagues, as mentioned above, but then you have some really good bowlers who 'sandbag'. Basically they'll figure out what is needed to win the games (start out strong, and see that they are beating the other teams) and then start throwing crap so that their average doesn't go up (and therefor the handicap goes down)
As the season progresses, they'll have built an artificially low average, so if they are playing one of the other tough teams they will really bowl (like they can) and then the handicap they've accumulated up to then gives them the edge. You would think people wouldn't really do that, but, you'd be wrong. Of course, not everyone does this, but moreso when there are money prizes to give out.
Been a long time since I've been on a league, but, I assume it hasn't changed in the leagues that have handicaps.