Kalista E doesn't scale with AD

I just tried Kalista on a custom and realized that her E does less damage than it should. I attack 3 times the golem, use E and do 350 damage. After that I look to the description and it says "dealing 60 (+216) damage. ... Each extra spear increases Rend's damage by 32 (+108)"Don't should it do 60 + 216 and then 2 extra spears * (32 + 108)? That will be 556, but if only counts the base damage per extra spear (32) it will be 340, +3% for masteries is 350. The golem had 12 armor and I had way more armor pen with youmuu etc, so that didn't affect the damage. Am I missing something or is it really a bug?