Why do people act like a Caribbean family can’t have a darker skinned , and most of the other lighter/fair skin tones in their family?

I’ve had people mostly white peoples ask me if that was my biological mother, nobody else hardly ever says that. People who aren’t ignorant and I guess educated have a better sense of it
Why do people act like a Caribbean family can’t have a darker skinned , and most of the other lighter/fair skin tones in their family?
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