It is the 'aspect ratio' that can be [re]set by various 3rd party cameras an other tools.
There are many camera options settable via the API.
Changing the image width/height so it doesn't correspond to the actual window size gives that effect....
And note the most other modelling apps doesn't have the type of viewport presentation as SU does. They usually display much cruder mesh and leave all presentation for the rendering part. While SU display presentation graphic straight into the viewport.
