Gary Coffman writes:

For example, the IR 5 hp dual stage I have is rated at 14.9 CFM @75
PSI, 14.8 CFM @125 PSI, and 14.7 CFM @175 PSI. Same compressor, same
displacement, same speed, and nearly the same CFM over a pressure
range of more than 2:1. If we were to believe they're talking about
CFM of pressurized air, the gas law tells us we would see more than a
2:1 change in CFM with a 2:1 change in pressure. But we don't. The
only way we can see the numbers they publish is if they're referring
to CFM at 1 standard pressure.

Exactly.