When is fullscreen not fullscreen.......

Take 2 photos with your iPhone, one vertically (portrait), one horizontally (landscape). Now view them in the viewer. View both vertically and horizontally by turning the phone as usual.

Notice how the one taken vertically fills the screen when viewed vertically as you would expect. But the one taken horizontally when viewed horizontally has black bars at the right and left sides.

Look at them in iPhoto and they are both 1200x1600 and 1600x1200.