I think the reason is because glancing does not count as a defensive ability. It absolute should, because it is only ifln effect when the champion is hit, but for some ridiculous reason it is not. If it was, then Falcon would have no problem shutting it down when locked on. I think it's just general ability accuracy. It's absurd and should be changed, but it's one of those things that has been the way it has been forever and I doubt will ever change.