Wouter, Rolf.

The "stiffness data" you are presenting says almost nothing about the stiffness of the hulls themselves, i.e. the difference between using wood or some modern high tec composite. It is nearly all about the other bits like beams, trampoline etc., in fact as Wouter ponted out its almost certainly to do with the beam connection to the hull. It is noticable that the older boats are less stiff. This is more likely to be wear than construction.