Why do you think people stare at darker skinned mom with fair babies?

I have a son who is 6 months old and he is biracial. Now I am brown complexioned but my son has light olive skin. Why is it that when we go out in public people stare at us funny. Now I know a lot of it is admiration bc my son is really cute bc people are always coming up to us and complimenting me on how cute he is. However at the same time it seems like people look at you odd when you have a child that is really fair in complexion as if a black woman cannot produce a fair baby. I mean what do they expect if the father of the child is white. What is your opinion on this?