I mean I never said it wasn't stupid, just that it's intuitive for Americans. 5/10 should be average.
But I'm not even sure that's purely an American thing. Go to a rating website like IMDB or MyAnimeList, 7/10 is considered average. I'm genuinely surprised that you're surprised by this.
As far as I can tell mesa and the proprietary drivers both use the ROCm packages for OpenCL. I don't think there's actually a difference on that front.