Sometimes as you move up in an amplifier or receiver line the next model may have a larger transformer or increased power supply capacitance resulting in greater headroom, even though the power output isn't appreciably different.
The weight of these two receivers is within a few ounces of each other, and I would guess the real differentiation is in the feature set. But marketing wise, manufacturers probably have to be able to quote an increased power output specification in the more expensive model, even if it is marginal, as in this case.
Steve