possibly.  i doubt it's that complicated, but possibly.  i think it's as simple as the iris is set significantly back from the scelera, and it has it's own (radially) uneven surface (why i usually see a shading differential towards the center).  how it's shaded seems, imho, perfectly consistent with all the diagrams i've ever seen (and the scale of the iris), so i'm not sure why it would need a really complicated explanation.  but perhaps you're right.

and in the end, why only matters so much.  for me, just personally, it would be a step backward to have a technically accurate eye that couldn't render accurately at all or needed burned in artifacts to look realistic.