I thought that most kitties tended to bite or scratch if you messed with their bellies. Those that don't, I thought, were the exception - exceptionally mild. Just my thoughts. As in - your kitty is normal. That is their most vulnerable spot, and they are instinctively protecting it - or - they get over-excited and bite. As you say, your cat is being gentle. Usually if my cat is biting me and it hurts, I stop what I am doing - and cry out a bit and say, "Stop!" and she stops. But if I try to pull back before I make her stop, she just claws and bites more - it is too much like a game or a hunting thing.