OK, is she a virgin? This sounds like a
hymen . This is nothing to worry about, some girls have one, some don't. If it doesn't hurt her then I wouldn't worry too much. Like Jean said, everyone is built a little different... she's probably never really looked at other girls there before so she doesn't have anything to compare it to. If it really bothers her, I would suggest having her talk to her doctor. The doc can take a look and make sure it's nothing to worry about and then can hopefully ease her mind.
It's not quite the same, but there's a video Dan and Jennifer have put out on the shape of a vagina being.. different...