I have asked mum and she says thats not true. its more to do with ur stomach muscles, something to do with them closing, if they join completely you'll have an innie, if they dont youll have an outie. I think thats probably because you can then lay fat down behind the belly button thus pushing it out, where as if they are closed and the skin is joined to the abdominal muscle walls, you'll have an outie.
I knew wondering about this sort of rubbish stuff would come in handy one day.