How they figure it out.
My 2 handed 6 weight takes a 425-500 grain line when casted overhead. I Use a 400gr line which basically would qualify as a 11w single handed rod. Now, I also use my standard, in my sense, 9 weight lines on this rod. My 9 weight lines are on the heavier side, 350gr. which I have many. Too many. parting with a couple now. (but that's for later, tackle junkie thread).
So my point, why would they just not rate the TFO pandion a, lets say 10w, instead of labeling it a 6W?
Point to ponder.