It's random lol.
Or, maybe the Primary typing is supposed to be the typing that suits the Pokémon more. Since a bat is more a flying thing, but also looks like a dragon, so it's Noivern (Ooh, these wings are too big). Charizard's pre-evolutions start with fire, and is mostly a fire dragon like thing, with wings. So that means Fire/Flying type, logic yay!