[GAMEPLAY] Kassadin do less damage to tower when w is active

Hello, I have tested this between customs and was realizing why i do less damage to towers with W active; in the W description it says that kassadin would do more damage with it active and I can see that when dealing with monsters or champs but nothing about having your passive being cancelled when hitting tower but if you don't activate it you do more, which doesn't make sense. As a kassadin player I find this disturbing.

Below my second pic i have gained 2 lvls because i had to save screenshots in between and pushing to tower. This is proof that below 2 levels i still do more.