Everyones made great suggestions so far, but i dread to tell you the following:
The reason you mightve gained only in BPEL is possibly bc youve gained some fat on your pubic region. So in essence, you have gained penis length (woot woot

) but it looks about the same in appearance because you also gained some pubic fat to cover what gains you have in the penis.
Hence NBPEL is what you SEE that sticks out of your hip region. BPEL is closer to actually what is THERE.
Example: I start with 8in bpel/7.5 nbpel.
I gain to 8.5 bpel but stay at 7.5 bpel. Why is this? I gained .5 in penis inches. BUT i gained 1 whole inch of pubic fat to cover it up. I HAVE a longer penis, i just cant SEE it, due to fat.
Get it?
Im not saying this is going to be the definitive reason why you have that condition, but it COULD be the case. Just a heads up.