Cylinder headchamber.
'302' heads are known as swirl type heads and promote better combustion and burn.

I wish I could say I had the same experience with the '302' heads but both on my previous stock 318 rollercam engine with 302s and the current 11.3:1cr 360 with polished 302s in my daily driver produce the same 'lousy' mileage of around 11-12mpg.
But I still suspect the A518 and its '60s torque convertor behind both engines being the main culprit here.