Why is that so surprising? You know how biologically diverse most African-Americans are, and probably British Blacks/Africans too (to a lesser extent).
The concept of "race" is a Eurocentric, 18th century invention. Technically, there is only one race: Humans. Different "races" like Blacks and Whites are Varities, not "Races". Biologically speaking, a Black man is more genetically similar to a White man than he is a Black woman. There is more genetic difference between genders than races.
On a personal note, my family is full of "mulattos". A few of my ancestors were Scotch-Irish, and I also have some Native and Thai in my background. Several relatives on my mother's side of the family have red hair, freckles and green, blue or gray eyes. Several relatives on my father's side of the family have Native American features. Both of my parents are brown-skinned, yet my older sister is brown-skinned while I turned out very light-skinned. Two of my cousins have dark-skinned parents, yet came out looking Biracial, having naturally straight-wavy red hair and gray eyes. Another of my uncles is light-skinned, and so is his wife, and all three of his kids are dark-skinned.