There are some other factors on credit earnings. If you track someone and another teammate issues damage, you will earn credits and XP for that damage. If you spot another tank and another teammate issues damage, you will earn more credits and XP for that damage as well. Someone else correct me if I'm wrong, but I seem to recall reading on the forums a while back that your credits and XP amounts that you earn are also determined by if you are damaging a tank of lower tier, equal tier or higher tier than yourself.