hmmm... is that an observation from your real life or from photos? It could be that..
A) the flash of the camera makes light brown skin brighter
B) photos of mixed colored children are more common in non-tropical places where they get less sun and are therefore lighter or
C) it's just an illusion. A friend of mine for example has a Russian mother and a Liberian father and he and his sister are pitch black (at least to what I am used to..).
It would be interesting to see how it looks in Brazil where you have plenty of these children.
PS: What I am always wondering is why 95% of these couples have a black man and a white woman. Are white men really so unpopular among black women?
Hahahahaha! Maybe whit men just want to stick with their race and preserve it. I don't know. Haha!
My observation is real life. I know many black and white couples whose children are mostly light in my area Africa.