The boxy shape would likely still be less aero in isolation, but certain parts that in its slipstream might be so un-aerodynamic that the see less drag through

The boxy shape would likely still be less aero in isolation, but certain parts that in its slipstream might be so un-aerodynamic that the see less drag through more turbulent air than through clean air, and the boxy one provides more turbulence. Or it's the reverse, total drag benefits from the boxy rear and the rounded one was round on the rear as well (or not round at all, just some knotty wrap that approximates "round" very roughly, contrasting with a clean box)