From Worms Knowledge Base
Your teams earn scores by every multiplayer game they play, no matter online or offline. You will not be given any scores if you quit in mid-game without having played all of the rounds the scheme requires. Scores are ultimately recorded as soon as the "match finished" screen appears (be careful with ALT+F4 quitting). The calculation of scores is as follows:
(won games * 6) - (lost games * 4) + (enemies killed * 4)
Note: It is about won and lost games, not each and every round. Scores are not individually calculated each game, the formula will be executed once only, not each game.
The formula has been proven false in certain situations that could not have been pinned-down yet.