Shield spell. Bug?

[Bacon_Cheese_Burger]

I have  GSF abjuation and when i cast the shield spell when wearing regular clothes, i will end up with +8 ac. +4 will be cast on the clothes and the last four will raise my base ac (i think) The problem is that when wearing a robe +1 and casting the spell it will only raise my ac by a total of 7. 3 on the armour wich wont stack with the +1. And the rest on myself.
This means i will end up with LOWER ac when in a +1 robe than in regular clothes.
I think the problem may have to do with the +1 on the robes not stacking with the +3 from the spell (wich is +4 when it is a regular robe)