Aftera (morning after pill)

Out of curiosity why is it some girls have spotting after taking a form of the morning after pill? Specifically if they've had their period a week prior before taking it. I know some people will debate the morning after pill being an emergency form of contraceptive or a form of abortion, but if you didn't have a light period or spotting after the pill does that mean sperm never fertilized an egg and if you do, is that I'm a sense a miscarriage or abortion? I'm curious as to why some girls who take plan B frequently will sometimes have spotting, but it's not their period.