The steel plate is one I gave her, I removed it before giving her the glass armor, she will not equip over her base armor.
Ok, then just substitute the word "steel" in my post for "default" and there ya go. For whatever reason, unless her base armor is light armor, she has enough skills and perks in heavy that whatever she is currently wearing is better than the Glass.
Remember, some of the perks are insane, +50% and more. It's easy for them to get high enough skill and perks in a skill tree so that their basic armor is better than a very fancy armor type in the other tree.