550 CI is the internal volume 87 or 88 is the capacity of air that fits inside 550 CI once you account for how air compresses above 3000 PSI
the inflated 97 or 100 cube feet is because they do simple math and do not account for the way air compresses above 3000 PSI
One dealer ignored the real calculation and the rest(the guy selling quality USA tanks) followed suit to not lose sales to the 1st dealer whom claimed to have a 74 Cubic Feet tank
You guess who started the questionable non standard statement of tank sizes they sold this brand of tanks
