a lot of it is. some groups are indistinguishable from the han, but are considered a different ethnicity, where as others are different from the han in almost every conceivable way (dress, religion, food, language, values, customs) yet they're considered han.
a country that has been conquered by outside forces so frequently, and had 4000 years of civil wars, fragmentation, etc, isn't a likely candidate for having 90% ethnic homogeneity. i mean, most of modern china wasn't even part of china historically.