Nice hitboxes

Hurtboxes (detects when your champion gets hit by an ability) are at a champion's feet.

Hooks have a sort of "lollipop" hitbox design, like this:

https://i.gyazo.com/6b828873bfb0bfe75e8bae5cd9dc1dea.png

(The black line is the path that the hook is travelling.)

So while it looks like the hook missed you, this is what actually happened:

https://i.gyazo.com/5f2fb245e2c0596553bdfe41a178f92c.png

The hook will then "snap" to you because the hitbox itself did hit you even though it looks like it should have missed. It's just a visual effect as DuskDaUmbreon said.

His tail is part of his hitbox. The hitboxes overlapped very clearly.

The hook moves to the center of Twitch because that's how every fucking effect like that looks. It's purely visual.

Nautilus' hook rotates while it's in the air. In the left picture, the hook is nearly vertical, so it seems thin, but the hitbox is more similar to how the how the hook appears when it's horizontal like in the right picture. If you imagine the hook in the left picture lying horizontal instead of nearly vertical, it's pretty obvious the hook hit.

With that said, there are indeed several problems in this game with how things visually appear in comparison to how they are.