>>45025029GameFreak has admitted in interviews that they sometimes give mons "incorrect" typings because they feel a gen has too much of one type, to differentiate a Pokemon from the rest, or even for "balance" reasons. Think all the Gen 3 mons with random Dark typings on them because GF was still scared of Psychics, or how Torkoal is missing its rock typing because it's in the same location as Magcargo which is already Fire/Rock and between Magcargo and Camerupt they didn't want a third Fire type with a quad weakness to water and grass.
Runerigus probably hit some internal "too many of a type" metric Gamefreak has or they thought the Gen was too unfavorable to Rock or something.
