I assumed it was just more of the 'suspicious midwife' thing-- not accepting at face value births not recorded within a year-- but then there's the requirement of parentage. Maybe they want to make sure that one "parent" didn't record births for a bunch of unrelated children? What do you think they want this information for?
Good question, but I went to school with a girl who listed 'unknown' for the father on her child's birth certificate. I wonder what that (now adult) child's experience will be when presenting that birth certificate.