"Fair and impartial" being a phrase does not preclude "impartial" and "fair" from having overlapping meanings. English has plenty of phrases like that. "Vim and vigor" are synonyms. "Intents and purposes", "prim and proper", "born and bred", "leaps and bounds", "pins and needles", "movers and shakers", "hack and slash", etc.

Every dictionary I've checked so far has "fair" as a synonym for "impartial" and a definition that is more or less "treating all parties equally" while none have listed "uninterested in the results" or anything akin to it.

https://www.merriam-webster.com/dictionary/impartial https://www.wordnik.com/words/impartial