Certain features are more common in certain ethnicities.

Certain features are more common in certain ethnicities.
But migrations between regions were more frequent than you imagine, even though East Asia was largely sedentary.
Warfares waged by nomadic tribes on China often forced Chinese to flee for safety, many ended up in Korea and Japan. Nomadic tribes themselves also mix with Chinese, Koreans and Japanese throughout history, swirling the genetic pool.
Koreans were occasionally captured by nomadic tribes and Japanese pirates and sent off to China or Japan as slaves or hard labor, eventually settling there.
Japanese pirates roaming the Chinese sea may also settle in foreign lands occasionally or leave descendants behind (rape or affairs).
Due to population exchanges, Chinese, Koreans and Japanese are all mixed to certain extent, hence many individuals may exhibit “atypical features” not conforming to the “ethnic norms”.