Why do some people think if a black person is not dark skinned they are biracial?

Okay I am the same complextion as Alicia Keys and people always think I am mixed but both of my parents are Steve Harvey. I am sick and tired of people thinking just because my skin color is not dark as Wesley Snipes I am biracial.If that is the case then white people with dark skin color such as Italians, Sicilaias, Spanish, Greeks should not be considered white.I read in an encylopedia that the black race has the most varety of skin complextions so to anyone who thinks that everyone who is ligt skinned is mixed or has white blood in them you need to get your facts straight.