The slider on the bar from what I've always assumed, is the default brightness you'd like to keep your device adjusted to in the the normal light setting you're in. When I go into a darker room, it adjusts, and the brightness lowers even more so it doesn't stand out as much. However when this happens, the slider does not adjust at all, it's just something that is automatically done, which why you don't see the slider moving back and forth to adjust. If you have a closet, or dark room you can try it out in, go for it. Set your slider all the way to the right so it's max brightness at that auto setting, and see if it tones down once you enter the dark room.
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