Kennen issue

So I was playing Kennen the other day on Howling Rift and noticed something about him. When he uses Lightning rush it does not give him the correct amount of movement speed. If you mouse over the move it says that he gains 335 movement speed. This is equal to his base movement speed meaning that after using this his movement speed should be 670. However, what I noticed is that it only buffs to about 580 or so. It also seems like after he hits a target his movement speed slows down.