Kennen E gives the wrong movement speed, how long has this been like this?

His E is supposed to give him 335 movement speed, he also has a base movement speed of 335.

335 + 335 = 670

So why is it that when he has 335 movement speed and uses his E, his speed only goes up to 565? It's 105 lower than it should be, that's quite a lot of speed.