@jbrat2219: Maybe because Kuu's lips are smaller and real? I don't know.
I think Kuu looks weird when she's with her mouth close, she doesn't even look like herself, and I think even she feels that way, since she poses with her mouth almost all the time.
This is a good example:
http://goo.gl/JkqVV2 x
http://goo.gl/NDxxuB
Anyway, this is an Ayu topic. haha