GSF Shield not applying correctly

[sharkinajar]

When casting Shield (I have GSF Abjuration) while wearing robes with +1 Armor AC, the spell gives only 6 ac, instead of 7 (8 - 1 from robes).

[Paha]

Got any deflection armor, like ring or cloak or something?

[sharkinajar]

None. And for clarification, the robes I am wearing are the Lucky Fisherman's.