This question can't be answered by people that aren't members of the MPAA.

No one but the MPAA knows the exact criteria they use to rate films.

As pointed out in the documentary This Film Is Not Yet Rated, The MPAA will not reveal any information about who or why certain decisions are made, and that the association will not even reveal to the filmmaker the specific scenes that need to be cut in order to get alternative rating [ Kirby Dick (2006-01-25). This Film is not Yet Rated (Film)].