HedgeHog is correct. Probably 99% of the world's humans have one side a little larger and stronger than the other. Even the face has one side different than the other side. There is a dominate side for everyone, like being right handed or left handed. You may not have been aware of if, but when using a BB, the dominate side will always take the majority of the work load. Even if you pay attention and try to avoid it, it can still happen. That's the reason HedgeHog suggested DB's for awhile. Even when reaching a stage where you think the sides are matched, one side will probably still be lacking a bit. Take a close look at the Pro's and you can see a slight to noticeable difference from side to side. Not to worry, it's human anatomy
Even if a person is ambidextrous, there may still be a matter of a few degrees where both sides are are exact in nature. Good Luck.