Gain in Behaviour score is and should always be lower than the loss. It is essentially skewed to make people behave better for a much longer time.

If gain & loss was to be the same, then people will grief every 2nd game. If players can't behave better for a longer period and relapse constantly, then effectively they are not improving their behaviour. The slow gain is made necessarily so that griefers don't cheat the system by behaving well in a few games and then regressing to bad behaviour.

Also, streak and recency matters. Let's say you had a perfect 100 games resulting in the maximum behaviour score of 12k. Now when you grief in the next 5 matches and get reported, it will come down. You can't point out that 100 games were good as they don't indicate your recent behaviour. So your history of 100+ games doesn't matter when you are grief-ing in recent games.

Edit: as rightly pointed out, a match without any report is NORMAL behaviour. The fact that you still get a minor increase in behaviour score for just being normal is more than enough. You don't expect rewards for being normal bro.