I think it may have to do with the image, Im sorry to say. I think your average 172 driver will be quite a few years younger than the average Cooper S driver (due to initial cost of car probably, image). And of course, insurers always penalise younger drivers because were less experience and apparently more likely to wrap it around a tree.
But I wouldve thought the Mini would be in a higher group as id expect the replacement cost of bits to be higher (being a BMW) hence crashes would in theory cost more.
Im 25, 2yrs NCB and going to be paying £785 with Elephant for my 172 - I didnt think that was too bad, but havent ever tried a quote for a Cooper S.