Do not be concerned about the imbalance. A skill deficit is most likely a significant contributor to this. Especially if you are a righty when you do a pistol with your left leg, you can make small adjustments with your right leg to stay in balance and not fall over. You are probably less coordinated in your left leg and less able to make the precise center of mass adjustments to stay inbalance and not fall over when your right leg is on the floor.