This is probably a dumb speculation, but this was my first thought knowing it's Apple -
Now that both the regular and Pro model come with 6.1" displays, is it possible that both are in fact of the same hardware?
Seeing that even budget phones have such displays, I don't think the cost difference between 60Hz and 120Hz panel would be significant enough. Also, wouldn't Samsung quote more to manufacture two (four including for Plus/Max models) different panels just for Apple? Different panels should add to some inefficiency in the assembling process as well. Given the scale of the operation, all this should add up.
Am I missing something?