I'd be willing to bet that scale was off by several hundred pounds..........or you own a factory freak. Factory curb weight for the Duster was closer to 3300 before fluids and accessories for V-8 cars. Even the Feather Duster with it's aluminum panels and aluminum S6 block was still significantly heavier than the 63-66 Valiant/Dart sedans.
That said, I really wish they hadn't built the 64-66 Barracuda's out of lead panels as compared to their Valiant sedan brethren. My 65 Barracuda weighs in at 3020# after significant weight removal: glass hood, aluminum heads, AlterKtion front end, rear seat removal, no spare tire or jack. It probably tipped the scales around 3500+ in stock trim. Nowhere as light as the sedan.