They're just synonyms. Neither of them are words of power (they have multiple syllables), but "unslaad" is much more commonly used. They're both canonical, so there's no arguing with them. This also means that Bethesda did it; it wasn't our decision.

Think about it like this; in English, the words "bracelet" and "wristlet" are synonyms, but "bracelet" is much more commonly used (nowadays, anyways).

Thanks!