I mean if your device or source video is output is in 16:9 aspect ratio type (Wide Screen 16:9) format. Then you have to set the same mode with in the splitcam program too. It won't change it for you automatically, just because you've changed or selected a different resolution. You have to do it manually with a click of your mouse button on one, or the other.
BTW -The "Square pixel" is for Standard 4:3 aspect ratio which means it's a non-widescreen its just a normal square viewed box shape, like older type tv screen box shape view.
On the other hand "Wide pixel" is 16:9 aspect ratio tells the program you want it to run in that mode, hence it will switch to that mode of operation. Since the source video is in that same wide format.