Why do two EV3 bricks have different power outputs? I have two identical box designs with EV3 bricks.  When I upload the same programs to both, one moves about twice as fast as the other. 

Is there a reason that this might be happening?  Is there anything I can do to update it?
Some possible reasons:


A motor is defective
One of the brick has lesser battery charge
Firmware problem: make sure both bricks have latest firmware