Why was my AC different after death?

I, Ebiby the Wizard, drank an unidentified potion (just for fun) and polymorphed into a Lich. Suddenly my AC jumped from 1 to -9 (I was wearing dwarven mythril armor despite its effect on my spellcasting). I took off my armor to see what it was with just the robe, which was -1. still seeming unhittable I proceeded without my armor. Eventually I went up against a Giant Ant. My spells kept missing it, so I tried to melee the ant, but it kept hitting me, which seemed odd. In the end it hit me more than I hit it and my spells didn't work, so I died after exhausting all of my potions. However, when I died, the attributes screen said my AC was 9! This explains why I was getting hit so much, but why was it different than what the status bar showed while I was alive?