I was asking myself the same thing. I did some reserch I think I understand it better now. I'm not a doctor or anything, but I been Jelqing and pumping for the last year and I did some good girth gain and about half inch in lenght.
The dick is made of spongy corpus that CAN be expended by the mean of breaking cell, dividing and multiplying.
But the urethra is made of a other kind of skin. It resemble of a elastic bone. It can stretch but to a point, and can't go futher. Since it harder then the spongy tissu it dont break cell and regrow and since it elastic it even less easy to break.
You don't want to break your corpus cavernosum (Urethra part). That would be like breaking in half what make your cock hard.
The lenght gain that people are talking about.
I thing it the ligament that hold the dick that been stretched. The same ligament that surgery cut to gain (about 1"). The same ligament that you straighten when you do kegel.
The lenght gain can be in part due by the bubble out of the head, it can be about 1/4".
But the lenght of the penis DON'T GROW.
So if a resume. The lenght gain is gain by having a bigger head, by maximising the blood flow (pre or to having poor blood flow) and stretching the ligament around the base of the penis.
May I underline here the fact that kegel exercice straighten thoses ligament and make it harder to stretch!
Maybe some hormones could influence the grow of the corpus cavernosum, but in anycase, it would affect softer skin before affecting elastic bones in my opinion.
It would explain why the flacid penis is longer, but don't grow beyond it maximum lenght.