I think 50% is the lighting. But I think the other 50% is the sharpness. The CGI is just too crisp. The edges are sharp and too clean. There's no softness to them. Case in point:
Notice with the neck of the real thing, there's a slight aura around it. With the CGI model, it's a crisp line that looks artificial. When there's a fake human or something closely resembling a human, there's a phenomenon called "Uncanny Valley". I'd say this is uncanny valley for a physical model. They're trying so hard to make it look like the real thing that it looks fake.
That particular CGI shot was greatly improved in BOBW Part II. I think they softened the model slightly and improved the lighting. It's much much closer to looking like the real thing.