I've given Lydia heavy armor, or actually any armor, but I haven't seen her put them on before, except her helmet. Does anybody know why?

lesser quality? she put on a full set of dwarven i made for her.

Got her wearing all dwarven armor ATM.

Bout to upgrade her to steel plate.

She was wearing Dwemer armor I made her, and now she is wearing Steel Plate that I found for her.