Background "overlay" setting changing the color of the background

This is another “not sure it’s a bug or just a quirk” issue, but:

The background overlay setting shows the background image normally when it’s set to 0 and when it’s set to 1. In between, though, the brightness of the background image goes up until around 0.25, then back down. Is that expected? I think it’s how it’s always worked, but I’m just now really noticing it as it’s making some parts of my current background image hard to see.

(Also, is there a reason that slider doesn’t increase linearly? Halfway across is like, 0.25)