I could see two ways it would work. Either A: it was a second defensive wych weapon (so there would be two offensive wych weapons, and 2 defensive wych weapons) that would have something to do with defense, so re-roll dodge save on the model, or lower the model base WS by one (kinda like what old ww did)
Or it could be another offensive one like a punch blade style weapon (aka... assassin creed blade) That could of been re-roll to wound, and the razorflail was re-roll to hit. They probably decided to combine the two into the razorflail to make it more competitive with the other two, and just kept the look because it is so memorable.