Cho comes to a wrong assumption that he's asking for it so that he can help her to her feet, that something specifically about her in a heap at his feet is what's setting him on edge. So she reaches up obediently. However, with Scaramouch'e height, and Cho's severe lack of it, she can not reach high enough to give him the ability to kiss her hand easily. She barely manages to reach high enough for him to grab it, never mind beyond that.