Let's Make Robots!

identical motors, different speed

Hi all, I have 6 identical motors, or so I thought until I powered them all on at the same time for the first time and realised they all spin at different speeds.

Question:

Has anyone else observed similar behaviour either when using multiple identical motors, and/or when using the Arduino Motor Shield?  

Background:

I am using the Arduino Ethernet + Motor Shield rev3, with 3 motors connected in parallel to Channel A and 3 motors connected in parallel to Channel B. Each motor is connected to a wheel.