I think it’s a lot like measuring vacuum. When I was involved with refrigeration and automotive mechanics, everyone measured vacuum in “inches of mercury”. No vacuum = barometric pressure, so the vacuum is 0” of mercury(Hg) below the barometer, as in a mercury filled, atmsopheric referenced manometer. A “good” vacuum was 28” of Hg, meaning 28” of mercury below the current barometric (atmospheric) pressure. That seemed to be a good enough system for auto mechanics and “old time” refrigeration techs. When I got involved in ultra high vacuum and medical(scientific) pressure and vacuum measurements, where it became painfully apparent that, the “system” I was accustomed to was not wrong, but just NOT VERY PRECISE, in that it was always referenced to the variable “current barometric pressure”. If the Barometer showed 29.5” “stormy” no one could ever go beyond a “vacuum” of 29.5”(below atmospheric) but if it was a sunny high pressure day )(BP=30.5”) it would take 30.5” to achieve the same kind of “vacuum” in terms of absolute pressure. So it is exactly like NPB, how much fat pad is there, how much was it “squashed” ? You cannot know, unless the measurement is referenced to a relatively stable and reproducible reference, i.e. pressing the fat pad, or BP measurements. Further, but probably way less significant way, is how much pressure is on the ruler for BP measurements. Most of us probably push quite hard so as to get the “best” BP measurement so it doesn’t make too much difference, but it could. But for NBP, some of us, like me, having a fair amount of body fat so we lose a lot of NBP to the fat pad. It is all a matter of frame of reference, only for PE, the reference is related to the individual’s fat pad thickness. So critically thinking, any measurement that does not give the measurement conditions (reference) is not likely to be very meaningful, in our frame of reference. It becomes apparent to anyone seriously involved in measurements or scientific analysis, that the “frame of reference” and measurement conditions are as important as the “raw measurement is. Hey, this isn’t rocket science, but just paying attention to details that can be significant.