Well, here is a partial explanation. The eyes developed as a sense organ attached to the midbrain and only later were wired to the forebrain. The midbrain retains complex circuits that we use in our reflexive behaviour. We use these circuits more than we would suspect eg while walking along corridors or while turning away from an object thrown towards the face. A stroke in the forebrain would leave these neural pathways intact. Hence the sight without concious vision.
Of course in reality these things are much more complex...
Of course in reality these things are much more complex...