The system provided the maximum attainable level for your character, given all the armor you had in your inventory and vault. Because a level 12 cannot equip light bearing armor, the maximum level a level 12 character could attain, is level 12. Another way to look at it - What is the highest level a character could attain, if you equipped your best equip able gear one day before the patch? That is the level you were given at the patch.