Only thing I can think of, is if the weapon has different stats for when used 1h or 2h, meaning that for example, used as 1h it has additional swing speed maybe, but as 2h more handling?
Still wouldn't make sense to me, because using it with 2h should make it better for everything I believe, but maybe that's why. I used to find that sword a lot in tournaments but haven't for a long while, never bothered to actually use it either.