How to learn the source of a ratings certificate
For a film that has multiple US ratings certificates listed on the Parents Guide page, such as Scenes of a Sexual Nature (2006) - IMDb , is there a way to find out where a rating came from, or what it is applicable to? This film shows R on the title page, which agrees with filmratings.com. In the guide both R and PG-13 are listed. A couple of vendors sell it on DVD with a PG-13 rating, but I'm assuming they're just applying their own opinion (which I agree with, but that's not the point). I haven't found any other references to alternate rated versions.
Is there a way to view the verification data supplied to IMDb for certificates in general? Or are vendor-defined ratings usually accepted? I'm just curious.