Class was introduced as a medium armour class that is might based, but all medium armour classes had been standardized to agility.
Months later, in a few quests, additional medium/might reward boxes were added to compensate. Beornings still had large droughts where they just needed to "make do".
Years later, the "solution" was to just make Beornings a heavy armour class. Done! No more problems, yes?
It was just not planned out very well from the start. I still like my bear, though, even if he now has to gather medium armour cosmetics so he does not look like a tin can on legs.