Here's an image of the frame of the hook landing before it's moved to your champ's center. http://i.imgur.com/swWZHnK.jpg It looks very very close. Could be a few things: Lag. Even low pings can make a difference in pixel-close hitboxes interactions. Your movement may make your own hitbox misleading, by having it extend a a little farther than it may seem. It's actually bugged. It's always a possibility, and with Riot, it's certainly not unheard of for things to randomly get changed like this.
Ping/UpdateRate/Camera angle