The reason is that on flights you are watching a specially edited inflight version. This has had the R rated parts removed by the movie studio so that it is no longer R rated.

Most international airlines have at least 1 or 2 pg-13/R rated movies. But this is out of a possible 30 odd channels (including audio music).

It's not too bad a ratio.

But these airlines have "personal screens".

Airlines do publish movie schedule upfront (1 month in advance usually). Not the same movies are shown in all the routes that, that particular airline flies.