Good question. I have a theory. Remember, this is based only on my experience and interest in mechanics, physics, science and physiology.
I believe that most men have some “stretch” in their penis and the exercises maximize this first. Once that is done the gains must come from either EXTRA stretch and/or new tissue growth. The benefit of extra stretch is questionable because the tissue must remain functional.
Anyone who has studied even basic biology should realize that permanent gains in actual tissue mass of 10% to 100% or more(length and girth gains) just isn’t realistic in a few weeks time. Actual muscle tissue (which the penis is not made of) will swell with fluid and glycogen and blood when trained but will shrink back fairly quickly with rest. Only after months and years of work will the tissue grow in true mass. I believe it is the same for the penis. The tissue initially stretches and then over time grows. The rate of growth depends on MANY variables but the theory is sound.
In my own experience I gained 3/4” in length within 10 weeks from flaccid stretching. I then worked on girth with jelking and gained 3/8”. Since that time I have gained very slightly but the gains have been solid. Good luck and dig in for the duration :)