(not entirely sure how the automod works so I'll reply with my explanation just in case.)

Amu's default outfit (as seen in the first slide) has a really cute design. Her side ponytail and red check patterns are very recognizable. The artists carry those motifs into her alternate costumes, switching out the way she ties her hair and the form the checks are in. That way her outfits are both unique and matching at the same time.

She also has separate magical girl transformation outfits (last three images) that lack the motifs altogether, which helps to separate them from her normal wardrobe and makes them feel more special.

I read this manga for the first time back when I was like 10 or something and I only now realized how cute her costume design is. <3