There's an official answer to this by the team:
63 used the formula [ N - 1 ], so SM127 uses the formula [ (N * 2) - 1 ]
They just followed the jig of how Super Mario 63 subtracted 1 from the original Super Mario 64. I guess they multiplied 64 by 2 (to symbolize this being a sequel) and did the same remove 1 thing, ending up with 127.