I think it’s the reverse shading compared to 12 that does this. In 13, you see that the slanted parts, the edges of the fader on the upper side and the lower side are shaded. In 12 it wasn’t like this.
Personally I can’t say that I find this new style of fader shading in 13 to be an improvement. Others downright detest this, we’ve seen the threads.
The reason why I don’t like this style too much is that it’s as if there is a small lamp somewhere near the focus of the parabola (the dip that the fader makes), so the inside part is very illuminated, and then the outer parts are illuminated by diffuse light. And this is for each single fader.
I don’t know if that’s the reason why my brain “rejects” this design, and if a different lighting would fool me better.
(For example, imagine if we had a led-strip on the very top of the mixconsole. Depending on the height of the fixture, we would see that the very top part of the fader would be very illuminated, the top part of the inside of the fader “dip” would be shaded, the bottom part of the dip would be illuminated, and the very bottom part of the fader would be shaded. And this would apply for any fader on the desk because the light source is stationary. Now, in 13, it’s as if each fader carries its own light source just over its center-line.)