How is Chetney’s Armor Class so high?

I gotta know, how is Chetney’s AC so high? I get the +1 to AC from the wolf form, but how does he have 18 AC as normal.
Loving the show by the way! His “Offhand Mallet” is a flavored shield, and he’s technically wearing a breastplate. 🙂

— Matthew Mercer (@matthewmercer) January 21, 2022