What makes a move that a Pokémon is immune to become a super effective move?

Question

Watch this replay
Why did Body Slam, a normal type move, become a super effective move against a ghost type, how is it possible that a Dragon type move is super effective against a Fairy type. Why some moves that are suppose to be super effective are either resistant or neutral, this is so weird.

Comments

Right at the first turn of the replay, you see Shadow Sneak literally hitting a Normal type

Answer 1

That battle was played under Inverse Battle rules, hence the "Inverse Mod" that appears in the list of rules at the beginning.