"Hi Hyungsoo, thanks for your reply.
Yes it was the problem of surface seam. To say in detail, it's because of the end points of the seam.
In my opinion, when the cutter touches the end points of seam, the result might have errors.
I really like your study about the hair field sway,could you share your definition of it?Or maybe you could explain it a little bit more?I want to model some something similar but I'm not a grasshopper guru as you are,so may you do me the favor? Thanks so much!