Yep, I think that's probably it. When have you seen a person with their head tilted one direction in an ID photo. When is the background any color other than white as well? I think the Ahmet Balkan sample ID at the top of the post would be the most accurate identification photo.
The other thing for me, I think, is the circle matches the shape of a person's face. Obviously I don't think the circle is a face, but it's easier to quickly resolve a circle inside a square, than a circle inside a circle.
(Don't get me wrong, it does look nicer when round.)
