A strange crit

Hey there, fellow tankadins

Today, I noticed something strange while tanking Black Morass Heroic - I was crit by the boss. However, I'm exactly at 490 defense. Was that a bug or did I miss something? Anyone got some explanation?

Thanks in advance

Educated guess
2nd boss + Improved SoCr
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R.I.P. Sanctity Aura, it's Hammer Time