Two things could be at play.
1. Recovery time. The water heater on gas will recover quicker than on electric. Therefore, you may have a little more volume of hot water on gas than on electric.
2. Hot water temperature. The gas and electric systems on the water heater are completely separate. One could be set at a higher temperature than the other. If the electric is set to a lower temperature than the gas, then it will take more hot water from the heater tank to produce a comfortable shower, thereby reducing the amount of hot water available.
