Kennen's lightning rush is too slow

when i was playing Kennen i noticed that his lightning rush was not providing the same amount of movement speed as the tooltip stated. With a movement speed of 357 i would activate lightning rush and my move speed would increase to 576. this is an increase of only 219 movement speed instead of the intended 335. I am unsure if this is a mistake with the tooltip or the ability itself but the two clearly do not match.