@McTag,
I think a stout rope, used to make a "cat", would commonly be made from three
spun strands, each composed of three small strands.
Hence, when you unravel the whipping end (that is, the business end because you would need a "whipping" or two on the handle (isn't English an awkward language?)) then you would have a whip of nine strands or tails.
Just a guess.