Same here. I think you'll find most people's favored arm has a slightly smaller upper arm measurement. My theory is your stronger arm is a lot harder to shock into growth than the other. Problem with hitting it harder is size and strength don't always coincide perfectly. So if you hit your right a lot harder to catch up, the offset in strenght might effect a lot of other things like bar curling, benching, pull ups, ect..... I guarantee you it isn't noticeble to others though, so probably better to accept it and live with it.
My right foot is an inch longer than my left. Now that's a bitch. I always get blisters on my left from having loose shoes.