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: 7
That's a good observation on your part and confirms what was said previously about speed and performance.