The area flat to the load takes most part of the load since it is furthest from center. Try to figure out why I-beams are shaped the way they are, it is the same principle. I-beams are stiffer than tubes for the same weight.
The trick of paper does not happen in a surfboard, since the laminate is bonded to the foam (and stringer).
Glue the paper to a piece of foam and do your test again 
wikipedia will also help explaining: http://en.wikipedia.org/wiki/I-beam
No the trick of paper flat that you rolled is not what happens in surfboards : section of surfboard is still a tube (more or less ovoïde) the only way you can increase quadratic momentum is by increase thickness of board.
When a tube work in flexion it’s material the farthest of neutral fiber, at top and bottom, that experiment more stress that’s way for the same size square tube is more resistant than a round tube in flexion because there are more material at max load point. So it’s the area flat to the load, farthest of neutral fiber, that take the load (one face in tension other in compression). Material perpendicular have to keep both face in place so it experiment compression and shear load so it must be calculate to not buckle. It can be the strengh limit for tube made with really thin material (like paper roll) because material are less resistant against shear and thin material in compression are subject to buckling easily. A square tube made with stiff material like glass/epoxy in flexion collapse by buckling of the flat side that take compression load then lateral face tears (because of shear and buckling).
thanks for all the replys
theres a few things that are going around my head with regards to this
if the carbon is on the bottom down the center it will mainly be in tenshion right?
if the deck has no carbon then the risk of buckeling will be no greater??
If the carbon is bonded properly to the bottom then would there actualy be any diferents in the amout of flex between the glass and carbon becase the glass is only going to move as much as the carbon does as the carbon is stiffer than the glass?
many thanks charlie
The stiffer the laminate, the higher the risk of buckling.
A stiffer laminate needs better support.
That is why I would prefer the carbon on the rails and not in the middle of the deck on a stringerless board.
Increase stiffness of skin increase buckling resistance. But buckling is a kind of flexion so quadratic momentum is as important as material stiffness. That s way a thick sandwich skin is stronger against buckling than a plain fiber skin of same weight. It s not a thin strip of carbon that will really change something, more cosmetic commercial trick.