"Why in the world would Eadric wear +5 mithral breastplate (+9 AC) instead of his +4 (+12 AC + invulnerability) Full Plate of Invulnerability??"
Sepulchrave II said:
Because of the max Dex bonus to AC - Eadric was under a heavily metamagicked cat's grace and his Dex was 20. He could benefit from the full +5 to AC, and gained a movement speed of 30 (60 expeditious). Mithril breastplate is considered light armour. DR 5/+1 was essentially traded off for an extra 10.'