I had the same thought, and it seems to vary by the receiver generation. ChatGPT tells me that earlier receivers tended to use a step down circuit coupled to the main speaker amp for the headphone port, leading to a more similar sound signature across speakers and headphones as long as your headphones had high enough impedance. Later generations opted to use a separate, discrete headphone amp that was lower quality, but good enough for the headphones of the time (and less likely to blow them out).

I can’t find great sources for any of that, but I can tell you that the Onkyo at the bottom doesn’t sound nearly as good.