Why would one backrest cushion of the dinette have a hard back on it and the other doesn't?
I don't know if your floor plan has the dinette pushed up against the driver or passenger seat in the cab but if so, there may be a cutout to allow the cab seat to recline. The plywood in the cushion compensates for this void. That's how mine is set up.
They use the same cushions that the 23XD does... in that floorplan, the one with the wood goes against the cutout that faces the cab, for extra support. We asked too.... :-)