I was just looking around, and realized that most of Sybill's predictions seem to have been correct. So I looked up the seer Cassandra, to see what I could find. It seems that in Greek mythology, Cassandra was cursed so that no one woud ever believe her predictions. Perhaps, this was handed down by generation and Sybill had the curse upon her. If this is true...PROPS to J.K Rowling for this brilliant link!