Oh geez. The backpanel power rating is NOT a max power rating unless it states it.
For more information, one can refer to the operating manual, HK does typically state in their instruction manual that the power consumption figure is a 'maximum'. They don't define maximum but just say maximum
Yamaha, on the other hand, depending on the model, sometimes they provide both 'maximum' power consumption and 'power consumption'.
For example, in the RX-Z7 manual, on the specifications page, they provided just 'power consumption' but for the RX-V3900, RX-V1800 etc., they provided the 'power consumption' of 500W and also the 'maximum' power consumption of 1100W, 6 ch and 6% THD.
As for HK, take the 254 as an example, the back panel says ac input 390W, and the manual says 540W maximum, 7 ch driven.
I don't think HK AVRs are rated conservatively. With the exception of their flag ship models, HTM, S&V, HCC etc. lab measurements consistently showed they rated their gear realistically, but not more conservatively than the likes of mid range AVRs such as Denon, Onkyo and Pioneer models.
If you compare Yamha and HK AVRs in the same price range you will find that in general Yamaha models have stronger amps while their power supplies are more or less equal.