Instead of showing 1:1 pixel, the KVM view has scrollbars and shows approximately 80% of the view. In other words you scale up the view by 125%.
Increasing windows scaling make the view proportionally more zoomed in.
Looks like you are incorrectly applying Windows Scaling factor to the video view when you shouldn’t.
Switching view to ”Adaptive” or “Best Picture Quality” sizes the image correctly but there is some blurriness. Maybe the view is scaled up and then down introducing quality loss.
"Original Pixel" is meant to display the original image size. If your original image is in 4K, it naturally cannot be displayed on a 4K monitor in a non-fullscreen mode without scrollbars.
This is because even though your main display has a physical 4K resolution, you have set a 125% display scaling. For the browser, the screen pixel value it detects (not the monitor’s physical pixels) is therefore not 4K. Only when you set the scaling to 100% will the browser’s rendered resolution match the physical display pixels.