Muted iPhone - sounds still coming through headphones

When I mute my iPhone with the switch, all sounds are muted, even app sounds. That's good.

But if I have headphones in, and the device is muted, I still hear app sounds in the headphones.

Why aren't they muted there as well? It makes no sense, and it means that if I'm listening to music in my headphones, it keeps getting interrupted by app sounds (whereas if I listen to music out loud while it's on vibrate/silent, it's not interrupted by app sounds, which is the correct behaviour)