Agreed! You can get a 20 gallon tank that is about 17 inches high and one that is about 13 inches high. Put a 30 watt light sitting on top of each tank, and the 13 inch high one will have about 70% higher light intensity than the 17 inch one. So, if you had 30 micromols of PAR (low light) on the 17 inch high one, you would have 50 micromols of PAR (high medium light) on the 13 inch high one, and it would need CO2 while the other one wouldn't. Both would have 1.5 watts per gallon.
This rule goes out the window even faster with nano or large tanks. It's impossible to quantify the rate of change of light waves with a volume measurement. That's like trying to convert sound waves to liters.