Yet I have used less reinforcement. 2 layers of 200grm top and bottom.
On my light wind board I have 600gm and a centre section of 1200grm glass
My other mutant has 600gm glass top and bottom and a stringer of 400gram carbon on the bottom also.
Yes i know.
But it is still a striking difference. considering I reduced reinforcement and reduced core stiffness considerably.
I'm not surprised at all! Carbon is 3-5 x stiffer than glass so your 400gsm (total) is an rough equivalent of 1200-2000gsm of glass (yadda yadda assuming same weave, resin ratio etc etc....)
as D mentioned, stiffness is a function of depth ^3 so increasing just from a 5mm core to 7mm (allowing the same 1mm laminate thickness & material and ignoring core stiffness) you are increasing the stiffness of the board by 1.77x