It's not imperial, because Britain doesn't do it this way. It's just America and American-dominated regions (the Caribbean for example). Instead of measuring a tank's capacity according to how much water can be poured into it, the way that France, Britain and most of the world does it, they measure the volume of air at "standard" conditions (sea level, 20c, etc) that it takes in filling it to its nominal rated pressure. If the tank is rated for over-pressure (a "+" sign after the pressure stamped on the shoulder) then that rated pressure will be 10% higher than the value stamped. So for example, if you have an aluminium tank stamped with 3300 psi (or more likely just 3300) and there is a plus sign after the 3300 then that tank does not reach its stated rated capacity until it has been pressurised to 3300+10% = 3630 psi. If the tank has a stated capacity of 80 cu.ft. then that will only be achieved at that pressure of 3630 psi. If it is in fact pumped to 3000 psi it will only contain 80 * 3000 / 3630 cu.ft., or just over 66 cu.ft. Rather over 3/4 of what you thought it contained.
This has nothing to do with the units chosen, so in the above example the pressure could be denoted in bar and the volume in litres. But as it is a practice only found in the American sector it makes sense for normal American units to be used.
