Even with his explicit permission to ask for actually useful information, she still focuses on the sleeves, doggedly so. But before he can be dismissive of a matter he views as trivial, she finally follows a logical train of thought and reveals an actual purpose to her question, besides pestering him. Though he may find the focus of her curiosity commonplace, it clearly isn't to her and she is correctly suspecting a deeper meaning. That's the sort of observation and analysis Lan Wangji does approve of and his features lose a little of their annoyed tension.

"Hindrance is desired. The Gusu Lan Clan chooses this manner of dress to encourage discipline and awareness in posture and movement." If he sounds like he's quoting from a textbook, that's because he very well might be. But he decides to reward her inquiry with a small bit of additional information and adds after a moment, "Also, they are enchanted for storage. Not here, though." Trying to store his guqin had been rather frustrating.