I guess the “one drop rule” didn’t apply to Native American ancestry the way it did to African ancestry.
J. Kehaulani Kauanui gives a good overview of how mixed-race White and Native American ancestry was viewed differently than mixed-race White and African ancestry in her book
Hawaiian Blood.
To put it basically, Native ancestry was seen as more readily assimilable to the dominant White culture because the Native population of the U.S. was small and thus Native ancestry couldn't overtake White ancestry if it were assimilated into White racial identity. Also, assimilating mixed-race Native people made it easier to deracinate and displace Native peoples from their land as the U.S. expanded westward and overtook more tribal lands.
Prevailing stereotypes of Black people as "submissive, obseqious, imitative'" and Native people as "'indomitable, courageous, and proud'" in the 19th century also factored into why "'it remained a popular truism that while "red" and "white" blood blended "quickly and easily" both resisted fusion with "black" blood'" (17). So a White person who said they had Native ancestry would still be considered white (and would have to prove somehow that they had Native ancestors in order to be seen as Native), while a White person who had any Black ancestors would be considered Black.
Blood quantum and the one-drop rule allowed for "'the simultaneous appropriation of Native American culture into and the exclusion of African American cultures from national culture'" (18).