I'm a bit out of my depth here, but what the hell.
Bodyworks gave a very good answer in the fact that it is just a temporary hormone inbalance which allows the body for a period of time to grow and hold onto additional muscle tissue.
What the body really wants to do is maintain homeostasis, and operate at a comfortable level.
I also don't think you would gain upto 15lbs after PCT on a 12 week test cycle - perhaps on a first cycle, but what % of that would be actual muscle?
Your blood plasma volume, glycogen levels and water retention would also be up on cycle, all of which would drop off after the cycle ended.
I think it took Dorian 5 years to gain a solid 25lbs (going from 225lbs to 250lbs) - and by solid I mean competition quality. I'm no Dorian expert so I could be wrong on the above.
Homeostasis - Wikipedia, the free encyclopedia