Why can some people get away with using copyrighted material on YouTube?

Whenever I make fan videos comprised of clips from movies they get removed, but there are so many other videos on YouTube with the same content but they don't get removed. I just really want to know why some people's stuff gets removed and others don't. Are the people using the right kind of video editing programs or what? Cause I know of some vidders who use clips that are ripped straight from the dvds and their videos don't get nailed for copyright infringement. What gives? Someone please let me know if there is some secret I don't know about or are these people just lucky? Thanks.