Why do people get called a fake for emerging in a culture they are not native to but resemble?

take this for example to use as a point i'm trying to make.Imagine if Bryton McClure(little richie from family matters) looks like one of the ways a puerto rican can look even though he is a biracial black and white african american.now say Bryton learned spanish and tried to imilate in puerto rican culture and spoke spanish with hispanics.There would people that knew he wasn't of hispanic origin calling him a fake puerto rican instead of commending him for his ability to fit in(by resemblance)and instead of giving him the commendation for having mastered spanish fluently.