It would be more relevant if grade recommendations in the manual were based on technical criteria and requirements, which unfortunately they are not. It’s the crux of the problem in any discussion.
I agree with you in many respects that there are probably multiple factors involved in oil recommendations.
Certainly engineering aspects play a role and a pretty significant one in discussions that I routinely have with manufacturing engineers as well as engineers from mobil 1.
But I have a feeling what you're getting at is the "other factors" such as country regulations (CAFE etc), business agreements (putting a product name on the oil fill ala Porsche and Ferrari does or putting a product name in the manual like Chevrolet Corvette and Porsche and Ferrari and others do) etc etc.
Still I don't think you can go wrong with contacting the OEM themselves to make sure you are doing the right thing.
Often times there are very good engineering reasons why the manuals differ from country to country. I certainly didn't mean to suggest that's always the case but I wouldn't want to guess.