I know Pokémon’s weaknesses, resistances, and immunities don’t always make sense, but what do you think is the deal with 1) Normal being immune to Ghost but Fighting isn’t, and 2) Steel being immune to Poison and Rock and/or Ground aren’t?
I think the Normal immunity to Ghost attacks comes from a sort of obliviousness to the paranormal. Ghosts and spirits have no hold over you if you don’t believe they can harm you, and Normal Pokémon are just too normal, too mundane, too out of touch with the supernatural, to know that they should even be scared. Steel and Poison… well, Rock and Ground do resist Poison attacks, despite not actually being immune to Poison attacks or the poison status, so I think we probably shouldn’t read too much into it. However, I suppose I would say that what I think they’re implying here is that Steel-types are more divorced from normal Pokémon biology than Rock-types are; Rock Pokémon have layers of anatomy that are in some sense “normal” underneath their silica-based shells, but the bodies of Steel Pokémon are radically different through-and-through at the level of their biochemistry, which makes them invulnerable to ordinary poisons.