We are very sensitive to event the smallest details about human eyes. There are some aspects that are not modeled in this demo. Probably the most important is

We are very sensitive to event the smallest details about human eyes. There are some aspects that are not modeled in this demo. Probably the most important is that head models are completely static: they were "frozen in time" from the time of 3D scan, in exactly one position, one facial expression, one eyes focus. Each of the models were captured with slightly different eyes orientation / position / focus. Black one especially doesn't look "at you", he looked "past you" when the capture was made. Demo only changes eyeballs orientations but the rest of the head stays always the same, mesh is static. In the real world when eyes move, also eyelids and skin around moves. We use these details as additional source of information when guessing where the person is looking at. Another aspect that's missing is a proper physiologically-correct cross-eyedness due to focus distance. When you look at the same point but at different distances, eyes will move closer/further apart, but not in a simple geometric way, there are physiological constrains.