Math Question regarding Damage Effectiveness

My Herald Of Ice at current level reads adds 28 to 37 cold dmg to spells.

My Ball Lightning says 30% Dmg Effectiveness.

I have 70% increased Ele Dmg from Passive Tree. 20% ele dmg from Mace.

I am computing, cold damage should be 15-21, however tooltip is showing 18-23.

I am using base * 0.3 (from dmg effectiveness) * 1.9 (from incr ele dmg from Passives + Mace)

Some not correct with my equation? I've checked my passives and gear a bunch of times. #'s look correct to me.

Just curious why my equation is wrong. I want to make sure I get the equation correct so I can see how it scales.

Any information would be greatly appreciated. Thanks.

That formula should work as far as I can tell. 18-23 would be the correct damage if you had 110% increased damage, not 90%. Are you sure you didn't miss some?