Why are there 2 sound icons on the touch bar?

Why are there 2 icons for sound on my 2015 Macbook Pro 15" Retina touch bar - one with 1-3 bars corresponding to the sound icon in the menu bar and the other is to the right, always with 4 bars and a strike-out line through it, in which the volume bars disappear when the volume icon in the menu bar is all the way to the left? Do they serve different functions?

Thanks, Richard