Schumacher was superstitious about odd numbers, not my bag but if that was what made him happy, and Rosberg didn't mind, what's the big deal. When Nakajima came into F1 he was given the number 4, which is unlucky in Japan, so was given the number 3 car to keep him happy. You will also note that there is no No.13 in F1 so it appears the FIA is superstitious also.