Varus' passive bonus

I've looked around the boards a bit and couldn't find the answer, so I figured I'd ask away (please feel free to point me at the right thread if I totally missed it). Am I correct in assuming that Varus' passive only gives the 20% and 40% bonus based on his base attack speed (ie, without accounting for any item bonuses)? For example, my attack speed (after bonuses) was 1.028. I killed a minion and the new attack speed was 1.160. Since 20% of 1.028 is obviously more than .132, it seems that the bonus is either a) coming from a different number or b) calculated incorrectly. I don't remember my level or base attack speed at that point, and if anyone needs screenshots or more specific info, just let me know and I'll go grab it.

Ultimately, though, if the bonus does only come from that base stat, maybe an updated tooltip to clarify that? Unless it's common knowledge and I'm way behind the curve; I did just buy him, after all.

Thanks very much!