This question is for my girlfriend.

She was diagnosed with cervical dysplasia(sp) just over two years ago. In September 2006 she had freezing done on her cervix to remove the abnormal cells. She went back every 6 months for check ups and every time everything was good. Her last appointment was in September 2008. Her gyno told her that her cervix looked "almost perfect", but he called her today telling her that her results showed abnormal cells again and she would need to come back in six months for another check up. Why would her pap smear come back abnormal after two years? What does this usually mean?