The real blood and guts of the design, is that it puts the centre of mass of the loaded box, directly over the mount.
Therefore there is no pitching moment as you encounter bumps. The arch shaped mount is aesthetically pleasing to my eye, but there is a more fundamental reason for the design…
Without that deep cutaway, you cannot get the damn seat off…! But it will go back far enough (just), to unhook the front from it’s mount.