That's exactly what OP is saying - chargers are commonly advertised as being able to supply a particular wattage, but that particular wattage is only attainable if the device being charged supports the maximum voltage the charger is capable of delivering.

OP is complaining that that leaves the true wattage of a given device/charger pair unknowable from the charger's packaging alone without further information as to what voltages and at what currents it can supply on request from the device. It's certainly a valid frustration.