League Of Legends Lee Q cancels Amumu ult if starting point is out

Exact moment when ult proc (Damage to Blitz & Lucian) : http://i.imgur.com/ql1waU8.jpg - Look exactly where Lee stands The same with the marks : http://i.imgur.com/BV9u5Px.jpg

Lee is clearly in, but the dash seems to completely ignore Amumu's ult.