Children are usually masters at mimicry and imitation. That is a very useful tool in evolution, as it strengthens the bonds between parents and child but also between family and child to ensure that the child is taken care of. So very often, you can see gestures and expressions, speech patterns etc that remind you strongly of this person and then suddenly of a completely other person from the other side of the family..
All this makes it far easier and likelier that the child is accepted as "one of us" and thus protected and taken care of by all sides.
When he was a child, Charles used to walk with his hands behind his back, in the same manner as Phillip did.
No idea, if there's any scientific basis to this, but I've been told that as a baby/early years the children tend to look more like their father. As said nature did this so that the father would (more likely) accept the child as his own and not think he has been fooled around with a cuckoo's child.