We go by non bone pressed here because that is the penis that is visible and often times we have no way of knowing how much exactly fat pad a performer has.
This is a very old and tired argument, in which nobody is going to change their minds 'around here', but given that "Buka_olly" appears new to the topic, I will add that there of a minority who (like me) have joined this thread to question the utility of opting for non-bone-pressed measurememt as a basline.
For some reason, questioning the 'science' of opting for NBP then implies to others that you believe silly porn fake measurements - whereas it IS possible to state that bone pressed measurement is the most universally/ scientifcally legitamised statistical basline, whilst at the same time recognising that at least 90% of professional/amatuer measured cock pics on the internet are rigged to deceive and exaggerate. These two issues (legitimate baseline and fakery) get conflated as one on this thread.
The strongest argument for using BP as a statistical basline is that the majotiy of clinical studies which have a serious role for sexual health and condom size, use BP along the top of the shaft. The common sense logic for doing so is that otherwise a man's penis size will fluctuate with his weight gain/loss.
The strongest counter-argument is that NBP is the "useable" penis, ie, ramming a hard ruler deep inside your body along the internal length of the shaft is like standing on tiptoe when getting your height measured.
It's not surprising that 'useablity' comes out on top (pun intented) on a forum dedicated to the study of porn, where aesthetics and pleasure are more important than acheving reliable industrury standards for condoms, but, the overly stringent standards seeminly applied by pseudo-forensic analysis of pics/vid captures (on this thread), usually go too far. (If a ruler grazes the fat pad then its an immediate 'fail' and at least 0.5 inches will be deducted, there are some habitual group think under-estimating).
Here's the thing - useablity does not have an exact marker which can be measured to the fraction of an inch. It is ridiculous to think otherwise. If you get into certain positions (during penetration), at the right angle, you can access some of that 'non-visible' penis, the simple physics of soft skin being pushed back by enough force. Yes, there is a question of degree, for many it is unlikely that they will be able to penetrate with sufficient force to reach all the way back to their pubic bone, but, conversley it's highly likely that anyone carrying some bodyfat that will have a 'useable' penis which exceeds his NBP measurement, in certain positions. The sexual position point is key.
With all those shades of grey, its entirely understandable why the science tends to stick with a BP statistical basline in the interests of consistency.
Carry on using NBP as the one and only meaningful measure 'here' knowing that is your 'preference' to do so - that's totally fine to make it your baseline, but in doing so I don't think that you argue persuasivly against what I say above.