Even with one amazing round, you still prove you have the ability to be a master. Now you are basically a master until there is a higher level.
I know what you are saying, though, becasue I am a 9 handicap in real life. But in theory I could I have a great and lucky round and shoot a -5..it could happen. But you also have to remember, real pros' scores can fluctuate easily by 10 shots...Tiger shot a 75 Sunday, but can obviously shoot a 63.