I always imagined it was just to flatten out the rocker along the center, but leave it in the rails. So when the board is "flat" a large portion of the running surface is flatter rocker, but when rolled onto the rail a majority of the running surface is now quite rockered. I've had two boards that were very similar, except one had a deep single and the other was flat. I think the concave board responds better to my style of surfing and pumps for speed more effectively, but the flat one goes better if I just stand there and minimize board movement. If you just stand on the concave one it jumps about a bit.
Boards shaped: 8
That's a good observation on your part and confirms what was said previously about speed and performance.
Ive made this experiment , 2 boards same lenght (6'6'' stubby shape) same rocker , same thickness, same fin placement, same rail profile. Board 1 has 3mm FOWARD VEE under front foot and board 2 has 3 mm SINGLE CONCAVE under front foot. They both runs into 3 mm double concave between fins.
They were tested in 2 - 3 foot beach break waves.
The FOWARD VEE board ride more smooth and is very pleasant to ride. It is slower and feels a little under water.Less speed out to a turn. Less quick reactions.
The SINGLE CONCAVE board is noticiable more quick, feels more ´´out of the water'' and is faster. More speed out of turns. Its a better board for small beach break tricky conditions.
It really feels that the single concave created lift .....the board felt more like ''flying'' on top of the water.
for stubby surfboard, flat with forward volume, i find that mostly flat bottom with double or channels back is an happy predictable shape. Now as boards come back to flatter, wider and thicker shapes i use less single concave, keep it on performance short boards.