When I first got my car back in the late 80's, it probably was a result child of the common practice back then. To register the weight lower as to get less taxed.
The regi read 3100 Lbs. It was way off on the light side by about 500 Lbs.
We got my friend "Glasses" 72' Duster down to a lean mean 2900 and change before he put a few comfort things back in. His was a 6ix cylinder from the factory. Wish the Feather duster items could fit other Duster years.