The answers above are correct but another way of looking at it is as mentioned: all lenses create a circular image. Most are designed so that the diameter of the image circle is greater than the diagonal of the sensor otherwise you are not filling the whole sensor.
Now fish eye lenses are designed to cover 180 degrees in all directions so to make the whole 180 degrees fit on the sensor the circle must cover no more than the shortest dimension of the sensor, which is top to bottom. So, you have a circular image that reaches from top to bottom of the sensor but not to the sides or corners.
One more part of the explanation: if the circle covered the entire sensor it would only cover the whole 180 degrees on the diagonals and not in any other direction.
OK did this help or did I get both of us hopelessly confused?