I've noticed that when I paint a portrait, the painting will start to look a little like me if I go for a while without looking at the subject (for instance, if the sitter leaves and I go on for a couple hours, painting, trying to work stuff out.)
I think this is pretty normal--we all have a pretty good idea of what we look like, and we're very familiar with other people who look like us, too. By which I mean, blood relatives.
So when the sitter leaves, it's perhaps natural to fill in the void with what we know, unless we're very consciously trying to avoid this. I think it's fair to say that I'm not the only painter subject to this phenomenon.
Perhaps that explains why some people (I'm not one of them) think Mona Lisa looks like Leonardo.