How did the MPAA grade movies in the 1970s?

I noticed movies like papillon and billy jack rated pg despite papillon showing a beheading and sexual assault and billy jack also showing sexual assault yet movies like texas chain saw massacre which had no blood and no nudity rated R? same with dirty harry some breif nudity scene and not much blood. Why was that? What was the MPAAs logic of the time? I understand some people might say "PG-13 was not a thing yet" which is very true however I dont see how that kept billy jack from an R rating...