I agree that it doesnt make sense to inflate the number to  million  Just a little insight into what I have found in Igbo culture  The reason why those in the diaspora are counted is because there are some known areas on the East Coast where Igbos who were taken into bondage during the Atlantic slave trade were taken  I think they are called Geeches and Gullahs  In Haiti there is at least one ethnic group which traces itself back to Igbos in Nigeria  So most Igbos have a strong sense of connectedness to other Igbos the Diaspora is counted in their society even if there is exact proof for it