The two common places you lose efficiency in a class-D is switching related losses and power dissipated in RON.
To accommodate the large VPVDD range (5.5V to 18V) there were certain trade-offs which resulted in a fairly large RON = 0.425, so heavier loads like 4 ohms will take a hit on efficiency as opposed to 8 Ohm loads which is about 90% efficient.