I believe the erevo motors have 15* advanced timing. So just running one in reverse, it would be slower than the other. Hopefully you just have the motors mixed up. If not you may have got two with advanced timing.

Here is a thread that supports my thoughts on the timing. Its funny cause Traxxas official says it doesn't but the motor part number shows it does.
http://monster.traxxas.com/showthread.php?t=433140


If you look at the motor positions in the Erevo you can see the motors run the same direction, that the reason they have the timing in those motors.