Where did you obtain your statistic? It is my understanding that 20% of African Americans have white ancestry and 30% of White Americans have a African American Ancestor. Of course, this dose not take into account Indian or Asian ancestry, but even if included, I doubt it would be as low as 30 percent.
Stats are from a mixed race political activism race. The Native American mix is very, very high in North America. Look at Mexico, It is by far the majority of the population there.
And in Oklahoma where I grew up, and indeed all of the cultural Southwest it is extremely common.