I think it has something to do with having stretched out the muscles at some time. The way it was explained to me is that the muscles sit inside a tough "sack" of tissue...once that gets stretched out then the muscle tissue may shrink again, but thats one barrier thats removed when the proper emphasis is placed on the muscle to induce growth again...which makes it easier for the muscles to regain the former size and shape. This is one of the reasons Dante emphasises extreme stretching in his Dogg Crapp training program.
So yeah, theres definitely something to the concept. I'm sure there are several reasons for it, but I know I've came back after a lay-off much faster than it took me to get there originally. I think the stretching thing helps, and I also think the hard part is figuring out how to get there in the first place...so once you start over that also is one barrier thats been removed.