very ver true, its the amount of air the car consumes but ya cant mneasure flow acuratly so ya measure presure, hence why i said what turbo

a ported head done well and decent cams will consume more air than standard and make it feel laggy but power at the top when the airs filling the engine

so if hes using a big turbo or got a well flowed head and cams he might be over speeding the turbo to a fooker to make the power at 17psi

or a leak in the boost pipe