I wonder when Japanese mangakas would discover the existence of chainmail and gambesons? They would fit perfectly in a medieval theme, Instead of this awfully impractical "armor" that covers only her tiny boobs and part of the rib cage, leaving everything else wide open.
or let's look at that leather armor that they that highly value? it got a "leather" word in the name, so it's literally telling you that it has close to zero defense against slicing and thrusting strikes. like, how a leather jacket would be useful if you are stabbed with a sword? and I did not mention that leather armor won't give you any resistance against blunt weapons impact.
so, its bad against slices and thrusts and awful against blunt strikes. BUT! +50 to style points! yep, worth buying!
and as timidhakana already said, a well-made set of full plate armor won't restrict your movements at all. you can run, jump or even do a barrel roll wearing it.
if you don't believe me, you can watch related to the question videos on youtube
edit: oh, they wore gambesons in the next chapter during a spar. so I have no idea why would shopkeeper suggest picking a leather jacket over a gambeson in terms of defense.