Yeah hawklore this is where our confusion came from also, but what straffo says is correct:
the beaufort scale is a way to describe the wind useing visual clues, so if you look out your porthole and see:
"High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over, Spray may affect visibility."
then you know is a force 9 Severe Gale outside.
Whereas the 'sea state' numbers and definitions directly describe the sea, totally ignoring what the wind is doing.
then I suppose swell confuses it even more as when does a 'wave' become 'swell' as thats gonna effect the numbers also.
all so confusing think rpm's describe it best