Same measurement metho, who different length.
NBPEL or No Bone Pressed Elongation Length is a measurement without pressing the measuring device (best is a stick ruler for a straight penis and a tailor tape for a curved penis) against your pelvic bone for your full erected penis size. It's barely touching your pubic skin to measure your penis. This is often called "observable length". The hidden part of the penis under your public fat pad may not be accounted for.
NBPFSL or No Bone Pressed Flaccid State Length. This is a measurement for your flaccid penis when it's rested a few minutes after your penis dropped from the erected state, with the measuring device barely touched the pubic skin. The best time to measure your flaccid state is after a nice hot shower when all tissues and muscles are fully relaxed. Give your penis a nice pull downward, wait for a 20 seconds, and measure.
You are measuring the penis the same way, with the measuring device placement at the top of your penis at the exact same spot each time, barely touching your pubic skin. One is to measure the erected size, the other is to measure the flaccid size.