Because without that title you could happily assume that the protagonist are actually supposed to be fifteen or older. The age of the leads is one of the key points, thirteen is teenage but we think of delinquency really kicking in just before adulthood. Not so, this period can last for years.
Kids going off the rails is not an uncommon theme in the cinema, but this version of the tale is notable for its even handedness. There are all sorts of aspects in the set-up which could be called forth as causes for the degeneracy. The mother’s (under control) alcoholism, the absent father, poverty and of course the apparently evil influence Evie. But the film does not want to settle for any or even all of these. Instead it almost shrugs it shoulders and says teenagers are strange beasts, do not even try to understand them. And whilst it ends at rock bottom it does leave the viewer with a small amount of hope.
A film with an excruciating, stay away topic (who wants to see ninety minutes of adolescence gone bad) ends up being riveting and surprisingly amusing. Fear the teenager. But see her.