Hello.
Archtor, welcome to Thunder and congratulation on your gains.
The main reason I believe this is so is because that P.E does not work for everyone. What I mean by this is, say you get a group of 20 persons, you ask them to perform the P.E exercises. After 3 months or so, you analyze the gains obtained from your subjects. What you will see is, (fictional figures) 30% gained an inch and the others not a lot and some not at all. So, conclusion P.E does not work.
But the thing is, P.E does work for all. The thing is that it takes more time for some than others. Some people gain easily others gain after a lengthy amount of time.
Cheers.
P.S: This is what I believe but I could be wrong.