Elemental Armor was reduced to 50% like the Spell Elemental Fortitude for the sake of balance, not allowing a single piece of armor to protect you from every element.

Elemental Resistances on Armor types do not stack if they are the same type if it's from an Item, however, if the Item's resistances are not coded to a spell the spell will stack with the Item.

The Burning Pearl is wonky. If you have some inconsistancies with it, it's because it's bugged.