The iPad Pro's display refresh rate changes on the fly depending on what's being viewed, which conserves battery life. If you're watching a movie or playing a game, for example, the refresh rate is at 120 Hz
I understand why games would run at 120 Hz, but why would it need to run at 120 Hz to watch a movie, unless you're watching a 120 fps movie (as far as I'm aware these are either extremely rare or don't even exist)? Wouldn't (commonly) 23.976/24 Hz or 29.97/30 Hz be more appropriate, and save battery?