Armor from Strength change?
Solution 1:
You have the enchantress as your follower, with the +15% armor buff.
Solution 2:
Every point of Strength provides one point of Armor. In the event you are gaining more than this, you are not accounting for something. Barbarians have Nerves of Steel, which allows Vitality to go towards Armor, too. Are you sure you are accounting for everything the item provides?