Iphone headphones volume very low on certain apps

I have the new in ear headphones from apple.

I don't know if it's fault of the headphones, but the volume for certain apps, especially calling, and podcast, is very low! I mean when I'm driving the sound of the engine or the wind is stronger and even if I use the phone external speaker I can hear more.

Some apps, like music apps have a slightly louder volume.

Is this normal? It's the headphone fault, or something related to ios and there's nothing to do about it?

Thanks