Your a half-breed; you weren't turned by Molag Bal. That's why your so much uglier than serana and even her father, Harkon.
That does not explain why one of the female nords who used to be with the companions has a clean face, she was turned, and for the male side, there is a wood elf that doesnt have that slit on his face, who was turned as well. It is a mess up on bethesdas part, thats it.