Explanation from Davids site:
"This analysis assumes that the New World (ie. the Americas) is made up of four ancestral groups - Sub-Saharan African, European, North Amerindian and South Amerindian. It ignores the rest of the world. So if your parents or grandparents are from Europe, and yet you score a couple per cent of Amerindian admixture, then clearly, that's not due to Amerindian ancestry, but some other influence taking its form, like North or East Eurasian"