Well the tolerance on the frames from manufacturers is about 8-9mm, as they are made in jigs and distort slightly from heat buildup when being tig welded.
When they are straightening frames, there is no heat which expands the frame, therefore they can accuratly set the frame in position within a 2-3mm, without the frame moving slightly when cooling.
Brand New bikes which are raced are stripped down and have there frames straightened, due to poor manufacture tolerances. :shock: