I had noticed this as well and the theory I came to was that the Application was scaling all videos to a fixed format (possibly 1080)
This is noticeable with with older low resolution videos e.g. 70's music videos in 4:3 aspect.
It can also be seen with newer 4K videos that are in a higher resolution/aspect ratio.
When played these will are also down scaled to a common resolution and if they are in a different aspect ratio they will be clipped to fit the display.
See below how Katy's face is distorted and the colours are a little washed because it appears to have been rescaled.
This would need to be confirmed by the developer but I haven't seen any option in zenpoint that instructs the image to not be rescaled.
Personally I would prefer to letterbox low res videos in the centre of the screen than to resize and distort.