1. I don’t think anyone’s solved the riddle yet, but there are some threads about it. I think it’s the skin at the base of the penis tenting or stretching outward, pushing forward the crease spot where you’d place a ruler for NBP measurements. So, not sure why, but it was the case for me where BPEL gained and NBP was stagnant or lagged behind considerably. I looked back through my logs and there were some stretches of months where BPFSL creeped forward, then BPEL caught up and NBP eventually later caught up as well, also some stretches where NBP stayed flat through a few months while BPEL showed gains.
2. It’s not about anyone caring necessarily, but using your pubic bone as a hard spot to place the ruler makes for consistent measurements to track gains without being affected as much by other variables such as weight gain/loss, diet or water retention, that skin tenting/stretching effect I mentioned, etc..
3. NBP is good to measure as well, but it can fluctuate a lot more than BPEL. You should probably also add in a BPFSL measurement, as that can commonly show gains prior to gains in either BPEL or NBP, so it can be a valuable measurement as well, especially for those times when you’re thinking of switching up a routine because there aren’t BPEL gains. Usually if I’m seeing BPFSL creeping forward, I keep going and most often BPEL will catch up. I think measuring all three can be beneficial as you track your progress and adjust your routines.