there can be a gene in the parent's chromozomes that does not affect them or even their parents, but that will trigger in a child. this is not uncommon. and i'm sure the genes related to this eye shape exists in many parts of the world, not just china. it could be in the native american population, in central europe etc.

my baby has this kinda of eye but she is not chinese her dad dosent have it n i dont think i do either