In some cases it's neither...it's brain structure.
Through about age seven the brain is still undergoing major wiring changes (some of which begin again at onset of puberty). Seeing is not simply optics. There is an interpretaional level where the brain learns how to compute things like vertical and horizontal lines, depth, and patterns. Much of this basic learning is structural guided by environmental input.
It's like "late-talkers", sometimes there's a problem, sometimes the brain just isn't wired for it yet. Many loquatious and brilliant people have been "late-talkers".