was checking on forum and no one posted this yet... the artifact say it increases defense by 15%, personally i was attacked by 8 adds in melee fight and only got 9%... after i saw this i switched artifact to eye of gemine anyway here is video from NU, hope it explains better then i did. https://www.youtube.com/watch?time_continue=9&v=oD7P55LJHn4
His math is wrong. The defense shown isn't your base defense. You have flat amounts of stats in the game and then % bonuses get applied to it additively. Aka If I had 1000 defense and two bonuses that added 100% defense each, I would be doing 1000 defense *3.0 not (1000 defense*2.0)*2.0) which makes a difference between 3000 defense calculated and 4000 defense. His starting defense is 58014. The fortified assault neck is a 10% defense buff. His passive as an ice tank is 65% defense buff. Which means his passive defense buff from the passives is 75% total. His base defense before %'s is actually X * 1.75 = 58014, where X = 33,150.857142. 33,150.857142 * 1.9 (75+15 from symbol) = 62,986.628571 defense which is where he was compared to the 66k number he originally estimated.