Destruction Staff passive Tri Focus is not working right

Mystrius_Archaion

Specifically, it says lightning staff heavy attacks damage nearby targets for 120%(or more) of the damage done, but I tested this and a nearby target dummy to the first only takes about 75%, if that, of the damage done to the primary target.
This doesn't seem right. Is this getting hit by double damage reduction or something to where it only takes the end damage to the primary target and then applies that to the other which is then reduced again?

This is broken.