"Minorities who are Asian" and "Asian" isn't the same thing -- the former refers to Asian-Americans, the latter refers to international (non-American) students from Asia. Also, the percentages reported by b-schools are defined differently depending on the school. For some, the "percentage of minorities" are reported as a percentage of the *entire* student body (US+international), whereas for other schools it's reported as a percentage of the Americans.
In any case, Darden may have less Asian-Americans than other schools (depending on whether that 5% is a 5% of the entire student body or just a percentage of the US students - if it's the former, they're not underrepresented; if it's the latter, they are underrepresented), but a good chunk (if not the majority) of the international students are from Asia, just like most b-schools.