Originally Posted by
Dubalicious
Well, the other poster almost answered this for you too!
Essentially, you have too much mastery when you are running into the "cap" often. So that is to say that a single heal, or perhaps your largest casted heal (Divine Light) followed by an instant (Holy Shock) is causing you to run into that cap, making a portion of your mastery not effective.
I don't think this is something you're going to run into very often, if at all, so I think it's fairly safe to say that there is no cap. (at least not one that is within reach!)