Ah, the video is where I was coming from. You should watch the video, but to paraphrase in the extreme, PG used to mean anything with content that parents might not want kids to see, and R was what R is, things rated for those above 17. This meant PG movies were included violence, language, and the occasional nudity (the seventies being what they were). In the eighties they created PG-13. Whereas the old system used to be "G is kid-friendly, PG is questionable, and R is just for adults". It became "G is for toddlers, PG is for kids, PG-13 is appropriate for teenagers, and R appropriate for adults". R began to cover things that PG used to cover, since we replaced the vague idea of "guidance needed" with specific age demographics (and because the eighties were what they were). As a result, movies began to cut things in order to fit PG-13. By being too specific, the rating system ceased being open to interpretation and became a thing that producers had to negotiate.