Let’s break it down.
Non Bone Pressed Erect Length is what you can see of your penis outward from your groin.
Bone Pressed Erect Length includes the part you cannot.
Bone Pressed Flaccid Stretched Length is how much you can pull your penis outward while soft.
NBPEL gains are the same usually as your BPEL. BPFSL often changes whether your erect lengths do or not. And only that has a correlation that isn’t always direct.
Bone pressed length and Not Bone pressed length typically gain the same. It also matters how you measure and what you are measuring for. Because lets say you have a good gain in a week or two and you gain 20 pounds over the next month. Your fat pad may increase so much you may not see your gains in your Not Bone Pressed Length. This is rare but possible.
Basically, in most cases If you have Bone Pressed Erect Length gains you will have Non Bone Pressed Erect Gains as well, equally.
Usually people start talking about the differences when they have a severe weight issue that causes an extremely large fat pad. So, it depends on your body type as well.