This is likely caused by the age difference, and the fact that they probably have some overlapping friends, even if it's not just one another. I'd have to guess the way it works is that Facebook looks at the poster, the poster's friends, and maybe the poster's friends of friends for a decent match among them when it encounters an untagged person. He probably scored higher because he does look like her, and those pictures are quite old and lack features that his mother currently has, such as wrinkles.
