I think that you are misunderstanding how AncestyDNA does testing. I linked how they do their testing. They test for a region.
Believe it or not; the Asante are some of the easiest people to test; because they can easily be tested against Brong and Baoule of the Ivory Coast and Ghana. The Asante; Baoule and Brong have the same genetic markers, because they are Akan people and they are all related and are from the Bonoman empire.
The Fulani and Yoruba would just be tied to Nigeria. The Yoruba would also be tied to Benin. Ibibio and Igbo are just going to be tied to Nigeria.
So they are not testing for a specific tribe; they are actually testing a region and they ascribe it to a country. Part of the testing goes back to questioning the Africans as to what tribe they belong to and how long they have been in the region.
So don't get caught up on the national borders. The scientist testing actually goes to region and a Yoruba person in Ghana is going to have the same markers as a Yoruba person in Benin and Nigeria.
AncestryDNA Regions