If you (the player) has recently just moved to Alola from Kanto, then you know all the first 151 pokémon from there, so when you encounter them and they have changed, they are different forms of the pokémon you knew back in Kanto. However, due to living in Kanto you wouldn't know any of the other pokémon from the following regions so you would mentally assume that they were all of their form when in Alola, so an Alola form for these mons isn't possible for the trainer to know, as they don't know what they looked like initially