Ok, this is the way it was explained to me. So if it's wrong don't shoot the messenger!
Taking AS will increase the androgen receptors in your body. These are the "things" that test and other AS bind too. Afterwards, you are left with elevated count of androgen receptors. So even when you go "natural" you'll get better results from your lower "natural" level of test compared to the same person who never juiced in the 1st place.
Again, don't shoot the messenger if it's wrong!