Here's mine: I think part of it has to do with fascia stretching. I think that is evidenced by past AS and AAS users(also experienced naturals) who even after a significant time seem to put on muscle easier than new or limited experienced lifters. That being said, I also believe it has as much to do with the central nervous system(neural pathways) becoming reaccustommed to an activity in which it learned before. This is comparative to learning to ride a bike. After you quit riding for a while, then resume, it is very easy to reaccustom yourself to the activity.
Any other thoughts or speculations?
I Just thought I would throw something else out there besides all of this fed-crackdown crap.