Someone called weights and measures on us once, because of this problem with a short fill. They came out, tested our scale, said it was fine, and left. They come out every year to test our equipment anyway (scale and the flow meter for RVs). It's not likely that the scale is the problem, or that you'll do anything but waste a bureaucrat's time.
I'd call the BBB if you think you're getting ripped off.
(We only charge by the pound, BTW, so we only charged him for the short-fill amount, but he was still pissed).