The segments of DNA I inherited from my maternal grandmother happened to be primarily the Somali portions of her ancestry, my father is also most likely 100% Somali, his contribution would further reinforce the Somali ancestry in my results.Because of your grandmother result, how come it does not assign you Ethiopian and Eritrean or at least Broadly Northern East African? Is it your direct grandmother from your family tree?
23andMe uses reference populations to estimate ancestry. If your DNA closely matches the Somali reference population, you may receive a 100% Somali result.