Plan B puzzlement

Within the past year, I have had to take Plan B twice (yeah, I know it's not ideal, but my boyfriend and I are going to start taking measures so that we don't have any more scares). However, I am kind of puzzled because Plan B hasn't messed up my cycle at all either time; it continues like clockwork. NOT that I'm complaining, because the only side effects I seemed to have were fatigue and moodiness. I'm confused because all of the women I've seen post about Plan B have cycles that are messed up because of it. Does anyone know why mine may not have been?