I think with a double tail, or twin tail, it technically HAS to be like two completely different tails coming off of the same rear end. :P Otherwise, like your guy, I guess it's a split tail, although I can't really find much online about split tails that doesn't lead to double tails.
could it be where she had a little nibble taken out of her tail and it's just never filled back in? i assume that part would grow at the same rate as the rest and so it may never completely even out?? who knows!?
either way she's super cute :]