As I have been told the tanks at any place the exchanges are filled on an line that is measured to be as close to the government minimum allowed to call it a tank of gas. If one goes to a refill place, TSC does it by the gallon not tank or pounds, they generally fill what the tank will hold, and that is quite a bit more than the exchange tanks. My TSC filled weighed a good bit more than the exchange tanks do and I filled mine for an average of $13 at TSC.
Every refill station I've been to used a scale.