Ian is absolutely right that sailor weight limits the upper wind range. From personal experience, I sailed last year on isonic 101 at 185 lbs. The stated upper limit of 8.5 meters was reasonably comfortable.
This spring at 195 lbs - only 10lb increase - the 8.5 is noticeably less comfortable.
It seems that when you're at the upper weight range for a particular board, even a few pounds can make a dramatic difference, especially in marginal conditions when you are pushing the sail size limits of the board.