lamont
Contributor
Scott in AK:OK I think I get it, you have to factor in the physical size of the tank, not just the max psi. The 26cf of "extra" gas is due to the max psi, not a bigger tank.
Actually its probably due to some combination of the incompressibility of the gas at higher pressures, differences in the physical water volume of the tank, and manufacturer's rounding off choices. Someone would need to do the work to accurately measure the water volume of a sample of both of those tanks to determine what the real truth is. In practice, its not a significant enough difference to care about. (If you are conservative, you should probably pick the HP tank value which is more conservative even if you're diving LP tanks, YMMV).