Why does it look like my retainers lose most of the gears benefits when I look to see which is an upgrade?

Let me try to put it another way. Say I have nothing in the glove slot on a retainer and I want her to wear something that has a defense of 10 and an mdefence of 20. When I hover my mouse over that to see the pop-up, I see that to equip it will give me a negative 10 on def and 21 on mdef.

But she has nothing in that slot so how does she end up with a negative amount like this? I know they have to wear *some* gear. How exactly does it work?