Where it is watched
Simulator
AI generated demo image
The bottom few percent of the picture, stepped. Whatever has vanished into the grey floor on the right is detail your audience will never see.
Why the grade looks wrong everywhere else
A grade is made on one screen in one room and watched on hundreds of others. Two things change it on the way: the curve the display assumes, and the light in the room falling on the glass.
Gamma 2.4 against 2.2
A grading suite works to a 2.4 display curve in a dark room. Most computers and phones assume something closer to 2.2. The same file on a 2.2 display looks lighter and flatter in the shadows, because the midtones and low tones are lifted.
Surround light is not a metaphor
Light in the room falls on the screen and is reflected back off it. That adds a fixed amount of brightness to every pixel, including the ones meant to be black, which compresses the bottom of the picture into nothing.
Black is whatever the room allows
A grading monitor in a dark suite might show black at a fraction of a nit. The same screen in a lit office shows black as whatever the room is reflecting. Everything you placed below that is simply gone.
Brighter is not better
A phone at 600 nits outdoors is not showing more of your grade, it is fighting the sun. The ratio between the screen and its surroundings is what decides how much you can see, not the absolute brightness.
Why the reference environment exists
A dark room, a calibrated display and a known surround are not fussiness. They are a fixed point: if everyone grades in the same conditions, decisions travel. Without one, every suite would make different pictures from the same footage.
What to do about it
Grade in the reference, then check in the worst case you expect: a laptop, a phone, a bright room. If the story point is in the shadows, make sure it survives the office. If it does not, move it up, or light it.