The Vagabond perk seems to account for innate, flat AC bonuses on armors, causing the total AC to be decreased if the armor has +1 AC because the innate bonus doesn't stack with Vagabond.
[hide=Working example - 3/4 leathers - gives +3 AC](https://i.imgur.com/6djsTEA.png)[/hide]
[hide=Bugged example - 3/4 leathers - gives +2 AC][(https://i.imgur.com/zNakPtF.png)[/hide]