tresh passive magic damage on hit

(Just to add on this (or otherwise just an inquiri on my part))* I first commented on a discussion called "tresh passive" but thought it might be better if i would just create my own toppic.

I get the feeling that the minimum amount of magic damage that the passive should give is different than that is noted in the skill, cause was playing a custom match and with a full build for magic damage on hit and 220 souls, in the case of my items all of them together would give me 169 magic damage on hit + the 220(souls) minimum should give me 389 magic damage on hit minimum but for some reason against a minion that has no magic resist i'm dealing an avarage of 250 magic damage on hit (once i start hitting with what i know for sure 224 as the lowest). Build: devourer(sated) jungle item, Bloodthirster, Wit's end, spirit vissage, hextech gunblade and nashor's tooth.

Can someone explain why the damage is lower than stated or what i'm missinterpreting here?

My thanks in advance.