I've noticed that happening to me as well, perhaps it adjusts its brightness level according to your surroundings. Pretty cool feature if it's intentional, and it doesn't bother me at all.
No, it brightens up so much because when it senses a lot of light, it makes the screen brighter so you can read and see the screen better.
Lower light, and shadows, makes it go back to it's normal light
Just try it, go up to a light bulb or outside, and let direct sun hit the phone or put the phone right next to the bulb, the phone/screen will go to max brightness. Use your finger to cover the sensor/LED area, and it goes back down to regular brightness.
I find it weird that there is an option to disable screen dimming, but none to disable screen brightening. I've had some instances where the light is kind of on the borderline, and it goes back and forth as I move around. It's usually not a big deal, though, and quite useful when you take it out in bright sunlight.
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