Pokémon - Pokemon the Movie X Y 2015 poster

The huge ring behind them is a portal. You know what's another name for a ring? A hoop! And you know what mysterious Pokemon's pokedex data mentions being able to warp large things through portals?

Hoopa! He's gonna be in this movie as a major character, I'm calling it right now!

This is pretty much confirmation that parallel universes do exist in the Pokemon World like Zinnia explained. So all the legendary Pokemon you catch from those rings in ORAS are pretty much from another universe, hence you can even reset something like Reshiram until its shiny, which you weren't able back then.