There are other factors, besides the ethnicity of the parents, that make the identity of a person. Bereket Simon parents are both "Tigrigna" ethnic group but he is not! He is just one example I'm giving you. There are millions like him who are not the same ethnicity as their parents. You see how his parent's origion is irrelevant then? If his parent's ethnicity or origin were relevant, he wouldn't be Ethiopian leader now, would he? He would be leading in Eritrea (or jailed with G15).
Just because you are the same ethnicity as your parents doesn't mean all people are the same ethnicity as their parents. By the way, this may not be easy to understand for some but I can point to you a lot of research done on this issue.
phsycology, way of life, including much of the language and accent is different.
There is more psychological, life style and accent differences within Tigray, say between Mekhoni and Axum, than between some Tigray parts, say Axum, and the "Tigrigna" of Eritrea.