This doesn't seem all that crazy to me. The Wikipedia article says that the left side of his face is paralyzed because the corresponding nerves are destroyed. The left side of his face may not wrinkle because the muscles do not contract. This is similar to the mechanism of Botox, which interferes with nerve terminals at the injection site.

EDIT: Also, one of the symptoms of Bell's palsy, which is a form of paralysis of the cranial nerves, is fewer wrinkles on the affected side of the face.