I think the reason for that is the body trying to stay in homostasis. Muscle growth requires anabolic activity to exceed catabolic activity. Once the receptors upregulate to accomidate the higher test level (and science nows tell us they do this, so until new evidence shows up, and it could, that shows otherwise, should we not say they probaby do?), the higher level does NOT become more anabolic. As you grow larger, your body does become more catabolic, so it becomes more difficult to gain new size. Eventually if you stay on the same dose (and do not find another means to increase anabolic activity or reduce catabolism), and have surpassed your natural limit, of course you will stop growing, once catabolic activity = anabolic activity.
Ung, all is forgiven, and you are correct about pot, as anywho who has smoked it can attest too. The same is true of alcohol.