Why did this xin zhao ult just do 400 damage?

https://gfycat.com/CheeryFeminineAsiantrumpetfish Lets do the math here. Xin Zhao ult has 75 base damage, 15% current health, and 100% BAD Xin Zhao in this clip has 157 AD, and Xin Zhao's Base AD at level 8 is 83.3, so he has a bonus AD of 74. The frame I take ult damage I have 747 health. So that would be an additional 112. Xin Zhao has no lethality or armor pen, and his ult damage adds up to 256. So, before resitances, I should have taken 256 damage, with 47 armor, I should have taken about 185~ish damage. But I took OVER DOUBLE that? Can someone explain that to me?
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