In that scenario, the brightness changes based on the room lighting.
That setting works using the ambient display sensor on a laptop. This behaviour is a dynamic brightness change that is unrelated to the 'Adaptive brightness' setting in Windows. These changes happen automatically after a couple of seconds, and they can also sometimes manifest as noticeable flickering. On a dark background, the screen will dim automatically, while on a bright background it will brighten. On some laptops, you may have noticed a change in brightness as you go from a screen with a dark background to a screen with a bright background.
