Ah, I did misread. I missed the "of the CO2" my mistake.Sort of a joke but sort of not- and what I said is true. Maybe you misunderstood.
The way those tanks are named is how much liquid CO2 they hold. A “20 lbs tank” holds about 20 lbs of CO2 when full, plus the weight of the tank itself. The stamped TW (or tare weight) on the tank is the weight of the tank empty. Sometimes they will also write on the weight of the CO2 it is tested to be able to hold, but it will be approximately the rated amount.