I think maybe having two abilities at once ? I don't want to put out the idea of Pokémon Fusion out there again, but it could be that but I highly doubt it :P
Maybe it has to do with something with Alchemy due to the starters and Legendaries are based off of Alchemy, so maybe they can change their "element" or type, because in Alchemy, you're trying to change the element or substance you currently have. Maybe with an item or an ability they are a different type (maybe have a different form ? ) or are not weak to the types they usually are weak to.
Because from the gameplay we've gotten so far, you get (what seems) the "mega bracelet" early on in the game right away after you get your starters (you seem to get it after your first battle with your rival) so the bracelet can contribute to Alola's Pokémon. :3