Kassadin's Autoattacks, with and without W active.

I was curious with how Kassadin's W is worded, and went into a custom game to test it. I noticed that when you don't have his W activated, he deals an additional 20 damage per hit, at all ranks, however, when you have it activated it deals 40/65/90/115/140 damage, but the damage from the passive part is not included per W active. Also, a thing to note is that when Kassadin attacks towers without his W active, he deals MORE damage because of the magic damage he gains on hit. Is this intentional?