Why do people think Im biracial?

I think people assume that all African Americans must have darker or a dark skin tone where it's not even the case. You can be African American and have extremely light skin, light hair and light eyes but that doesn't mean your parents are different races. I think people are just too quick to assume. I am just like you, having light skin, long dark hair, dark eyes. People have assumed I wasn't even related to my parents once and someone even asked my sister who is darker than me if we had the same father. I just am light skin because that is my skin tone.