the iris is slightly inset from the sclera.  in real life, shadows have no bias.  so that slight inset means that there is a darker area on the iris underneath reflections/highlights, a lighter area opposite it, and a shadow from the eyelid.  and the iris looks the way it does from the side mainly because of refraction. 

people make the iris concave because most renderers can't render an eye correctly.  i once saw an eye doctor of some sort try to get a properly structured eye to render properly in Poser and completely fail due to problems with how Poser renders refraction.  that was a P5 or P6, iirc.  if anyone can, bagginsbill can, though i'm just not sure it's possible.

at one point i had a bunch of photo references showing the shadow/highlight effect.  i don't now, but if you have a nice directional light and a mirror, you'll be able to observe it in your own eyes.  oddly, you can also see it in how people draw eyes in cartoons.  the benefit to looking a toon is that you can see the different shading zones in a simplified shapes.  it's the reason V4's eye's look like doll's eyes, where the more deeply inset Unimesh eyes have more "depth."

Eye cross section diagram