It's like that on Google Pixel phones too. Even if you enable the 120 Hz refresh rate, it'll only each that in certain situations and be lower most of the time. Seems like a good tradeoff for increased battery life.
The video shows the iPhone mostly running at 80Hz when scrolling. Pixel phones (and most Android phones) stay at a constant 120Hz until the movement stops. See: https://www.youtube.com/watch?v=Tbb0SoH765s