I think you should check your displays scaling settings. It is probably overscanning.
If you set the mask brightness to max on the OSSC and increase v.mask until you see a white line appear you can see how many pixels are missing.
Most displays will scale the 1280×1024 image to their native resolution ex. 1080p. If you don’t want your displays to scale the image then it must have ‘Original’ or 1:1 scaling options.