Anytime a submarine gets credited with more than 1/4 total of battle damage, it actually reflects upon poor team coordination/participation by fellow team mates.
(When everyone in the team I am on is fighting the enemy, I am lucky [in a SS] to average 5% to 10% of total battle damage)
This also an excellent indicator of fellow team mates (Bravo or Alpha) A-B ing in support of the opposing team.
Not necessarily... It depends...
If you, as an SS, run into enemy BB5s cluster with no enemy ASW around, you sink 3 of them you get 180k damage; then the last 30k damager may come from a CV.