Because "race" in America is conducted based on the color of your skin and nothing else, this leaves a large portion of it up to subjectivity. A person with "dark" skin is considered black\African American it doesn't matter to whom their parents are, just the color of the skin. In theory because skin color can be different among siblings (brothers and sisters) you could have 2 people who would be categorized as different "races" despite having the same parents (though this would be described as many as a "edge case" it does exist and brings this all into question).

Interestingly enough, there are some Native American tribes with heavy white and black influence which causes them to be miscategorized as they don't "appear" "Native American"\"red skinned".