About the listen.moe volume slider

So this is what the listen.moe homepage looks like when a song is playing:

When your mouse hovers on the volume button, a slider appears beneath it, like this:
listen.moe's volume slider
On this slider, up is louder. Makes sense, right?

Well, not really, I think that in this case, down should be louder.

The waveforms on the left start from the bottom of the player bar, and so does the volume slider.
Other volume sliders usually follow either up is louder for those that extend upwards, and right is louder if they extend to the right.
This would mean that if the volume slider extends downwards, down is louder.

And it’s very awkward to raise the volume: you need to hover the button, go down, and then drag the slider back up. Compare this to the usual motion which is hover, move in the direction of extension, and drag further away.