There'll be something in this regarding humans being naturally attracted to "average faces" as in average placement, symmetry and size of eyes, nose, mouth, etc and, for example, Hollywood actors, frequently being in the business due to being attractive (not to say they lack skills of course...).
Therefore with celebs, as they are generally attractive people, they will be grouped around this 'average' centre and so you might be dealing with a lower range, with many then looking alike.
If you can see what I'm getting at...