you should have asked them why they can't set the rear inside tires in the extra 1.5 inches at the factory

I'm not sure, but it looks like the frame that carries the tractor along the assembly line might not allow the tires to go in any farther.  It looked like in one of the photos there are shades of black rubber (tire) markings on one of the frame legs supporting the tractor in the first set of pictures (where the cab first appeared [picture gk5]) suggesting they may put the tires in as far as the frame will allow them to. 

There may be a safety factor with the width of that frame carrier when the tractor is "up in the air" (before tires/wheels), ie. if they made it any narrower, they might not like the stability or lack thereof when adding components to something sitting that high on that narrow of a perch.