Without knowing much about serious tea tasting, I'd say that professional tasters judge bitterness by what part of the tongue/mouth/throat is most excited by the tea. In high school science I learned that different parts of the tongue react to bitter, sweet, sour, and ... something else.
I know what you mean, though. My friends and I can never agree on what a bitter beer is. And that's a really important topic :)
Gavin