Drivers, usually, want as wider angle reversing cameras as possible. However, manufacturers understand this and therefore exaggerate the statistics.
You can measure the viewing angle of reversing cameras horizontally, vertically or diagonally.
The viewing angle which reversing cameras produce and the display might not be exactly the same – edge etc can be missed off to fit the screen. When the viewing angle increases the straight line distort and curve.
If you go past 120 degrees the distortion becomes terrible and distances become more difficult to judge – this is known as a fish eye lens. The viewing angle can be worked out from the focal length of the reversing camera lens and the sensor size.