For the most part, the dominant genetic traits within a family can leave a fairly clear road map for how a new baby is going to turn out.
But while this can often result in children who look a lot like their parents, it's pretty hard to predict how exactly the genetic information of two families will end up mashing together to make one of us. And that's not even taking into account that each of those families has some rarely-expressed genetic features that can potentially crop up in any new birth.
So while that goes a long way to explain the little bodily differences that we're about to see, it also makes it easy to marvel at how rarely we see them.