“Thematic Elements”

I notice that some movies, which are rated PG-13, indicate that one of the reasons for the rating is “thematic elements”. Does anyone know what “thematic elements” means? Are stories no longer supposed to have themes? If a movie does have a theme, does that somehow necessitate a stronger rating? If I were a parent, I would think a movie with “thematic elements” was a good thing. Books have thematic elements. A Tale of Two Citites has thematic elements.

What gives? I just don’t get it.