Answer 1

It's just a quirk on Gamefreak's part.

None of the other Rotom forms change their ability, so I guess they just left Rotom-Fan with the same ability for uniformity, even though it's useless because of its typing.

Comments

thats not true, if moves like gastro acid or mold breaker pokemons came into battlefield against it, it'll still be immune to ground types c: therefore no ground weakness

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Are you saying that levitate isn't useless? Because if so, that's... not making sense. Mold breaker on Rotom-S doesn't do anything. Regardless of whether Rotom-S had levitate or not, it's immune to ground due to its Flying type. The ability *is* redundant, because its typing already grants it the same immunity its ability does.

Levitate or not, Rotom-S ain't getting hit by Earthquake unless you use Smack Down or give it an iron Ball.

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It sure ain't :P

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thats what i'm trying to say D: even if someone cancels its levitate ability, its still immune to ground types therefore no ground weakness

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But you said "thats not true"...? I don't know what you were disputing. :x