Class Skill Mystery

Anyone know why a Skill that was not a class skill at character creation suddenyl becomes a class skill a few levels later?

I can only see two reasons: either the character is multiclassed (but that’s probably not the case here) or the skill was given in a module (through a conversation, or as a reward for a quest successfully completed).
I’m quite sure wearing an item giving extra ranks in any skill can’t grant said skill to the character.

Actually, neither of those things have occurred. Simply was not a class skill one level, then was at the next level. I have checked every feat granted to this class and none of them have anything to do with granting the Craft Armor skill as a class feat when it clearly shows in the cls_skill.2da that it is not a skill. Any script for any feat given to the class does not even hint at providing this skill. So, I am still at a loss.