Dude, are we sure this maid ain't doing it on purpose? I mean, where did she get this armored apron from?
She is absolutely not doing it on purpose. The almost every time before this, any changes to her outfit were impositions from the lady of the house, usually attempts to discourage her son's attraction to the maid only to awaken yet another new fetish in him.
Actually a lot of knightly armor, particularly suits meant to be worn on horseback, wouldn’t have armor on the butt or back of the thighs since on a horse those parts aren’t exposed, and they weren’t expected to need to get off their horse until the battle was done. They also sometimes had very long spikes on the toes of their sabatons for decoration, since they weren’t going to need to walk anyway.
The cuirass would still have gone all the way around though