Why do slim or skinny girls think they're fat?

It's insane I see nice little skinny slim girls on fb commenting on s nother. Girls pic who is kind of slim and she says she is jealous of her skinnyness and that is she is fat when she is like stick skinny! So many girls think a little roll or stretch is bad its frickin amazing how many girls actually starve themselves because they're too "fat" and they're as thin as paper