In the American south, some of those claims of Native American ancestry are borne of racism.
There is a lot of African American ancestry in the white American genome of the South. Lots of black folks have a white great granddaddy, and lots of white folks have a black great granny who could "pass."
This was absolutely unacceptable to the whites involved and so it became a common thing to instead claim a Native ancestor to explain the darker skin.
So, some folks who think they have a Cherokee grandmother actually have an African American one.
The more you know...