In my experience with cost-optimized low-end LCD displays, changing brightness settings will more or less alter absolute contrast at the lower end as well.
Even if the ratio stays the same, the darker shades suddenly are a lot more distinct as they just aren't consumed by artifacts and color shifts or even just glare anymore.
(Brightness is the adjustment that decreases the max white level without also making blacks brighter. So, a display that only has a contrast knob would count as missing a brightness adjustment.)