It’s actually three tiers: Scottish students get to go tuition free in Scotland (as opposed to paying tuition at English universities) so it’s ultra-selective for them. It’s highly selective for the rest of the U.K. sort of like Durham (posh students who didn’t get into Oxbridge) with the added aura of royal connections (William & Kate) and a nicer area than NE England. Then you have the Americans who pay for the rest of them and have much less stringent admission standards (and I’d guess make up much of the bottom half of the class).
I also don’t think UCAS tariffs tell the whole story, my niece with the three A*s didn’t even apply to Oxbridge because one of her A levels was “non-core” for her subject (IIRC it was art) and environmental science is a more specialized degree so not an Oxbridge subject. Same for my nephew (doing maths, physics and geography when you need to do chemistry if you want to be considered for Oxbridge). I don’t think St Andrews is necessarily quite so picky about subject choice.