It's amazing to me that they still have the flexibility to add features this late in the game for a product that has to be manufactured in the millions. Shows how streamlined and efficient their operations/supply chains are.
It depends when the production actually started for which model. If you look at the Apple iPhone typical production, it usually commences in mid July when it starts mass production, but given that there’s a staggered launch in all likeliness this year, the 120Hz Display is exclusive to the ‘Pro’ model, which is the later model to be launched away from the ‘entry level‘ iPhones.