Completely agree. It's probably largely because it's Britain, and despite the migration of recent years, they don't have the experience of large scale admixture. Even here in the U.S. we've seen or read of children of "mixed" ancestry who look predominately, even exclusively like one parent rather than the other.
There was a famous case about which a book was written where a seemingly "white" couple gave birth not to a black child, but to a very obviously "mixed" looking child. It was tragic for everyone. The father doubted it was his, and got a paternity test. When it turned out he was indeed the father, the mother was "blamed". She must have had part black "hidden" ancestry, largely because she and her family weren't extremely fair.
Well, it turned out that the "hidden" ancestry was on his father's side. The grandfather was more than a quarter black, but had hidden it all his life, and had gotten away with it because it absolutely didn't show at all. Well, all those genes showed up in the grandson: he was the one that looked quadroon. I posted about it here, to the usual caterwauling of protest that it was impossible.
We also know of a lot of cases of "passing", where one sibling could and did pass, and the other ones could not.