[ahartnet]

It could be mechanical as others have indicated (chain being tighter one one side, one sides gearbox not being assembled as smoothly as the other, etc), but it could also be electrical - if one of the motor calibrations are way off for some reason, that could be an indicator. Read up in the user guides of the motor controller for how to calibrate.

Also are you using arcade drive (pushing forward on one joystick makes both sides go forward?) Otherwise it might be user input making it veer off a little bit.