Is this not what they call double hollow? They use 2 different size wheels when they grind it. A large wheel makes the first cut and they come back with a smaller wheel near the spine leaving that belly for strength along the length of the blade. It gives the blade some slight flex while you use it.

I imagine it working to help keep an even pressure on the skin while making a pass. Allowing for slight changes in the angle of the blade by the user and helping it stay proper. Now that is just >me< thinking right now but I have read about this a few times before. Being a Dubl Duck user I have always thought that that name was descriptive and not just because of the 2 ducks on the tang :<0)