Should be obvious, but the top image is not crescented, the bottom image is. In my opinion it helps establish a 3 dimensional effect to the iris. From my understanding, it is possible, but difficult, to achieve the effect using studio lighting.
Occasionally, if you watch people from a profile view in subdued lighting, with a light source on the opposite side, you can see it in action.
Point is, it should be opposite of the catchlight in a real life situation, but in a lot of images I see the effect always at the bottom of the eye.