An unfortunate example of poor management as far as I am concerned. The axle is believed to have been moved forward at some point in order to combat axle tramp, which was later resolved by fitting tramp rods. Nobody seems to have thought it necessary to alter the position of the wheel arch in the rear mudguards. The end result is something of the order of 20 years production of trad Morgans with the rear wheel offset towards the front of the wheel arch. If you asked someone "why?" they would probably tell you that the mudguards were always done that way.
If you measure the actual width of the rear guards, from the tub out, you will almost definitely find that they are different one side from the other. Likewise there should not be a difference in the fore and aft position of the guards but the fact that there is does not surprise me greatly. The joys of ownership of handbuilt cars, particularly when they are built in a workshop where a good number of components are fitted and placed by eye instead of using accurate jigs.