Studded leather not granting all the AC it should

[Draugir]

I have 14 tumble, I put on a studded leather armor that grants base 3 AC + 1 AC armor bonus. My tumble is now 13, but screen says my AC is decreased, so I only get +3 AC from equipping said armor

Let me just add, that this doesn't happen all the time, sometimes I actually get full AC from the armor? So it's definitely a bug, unless someone want to check it out?

playername: Draugir101
character: Kruvor