Here and in Europe the 6 come from stock in R16x7 with 205/55 (not 60) and R17x7 with 215/45 (not 50)The American car-buying public likes cushier rides -hence the greater sidewall.
I believe the two available optional wheel/tire sizes in Europe (in 15 and 16) match the 215/45-17 ratio better. And that the car's gearing is designed to use those.
I don't doubt they've recalibrated the speedos installed on North American 6's for 215/50-17 and 205/60-16, though![/b]