Twitch crit for 1680 with only Infinity Edge???????????

Infinity edge was his only damage item and he somehow crit for 1680. How is that even possible? No, I'm not mad about it because as you can see, he was on my team. I am just curious as to what mechanic allows him to crit for 1680 with about 200 AD. A crit with 200 ad should do 500 damage if the enemy has 0 armor.