Hi,
Typically the recommended resolution from Windows matches the native resolution (i.e. the actual number of pixels on the screen) so something seems to be strange about your setup. I’d recommend checking that your graphic drivers are up to date, and verifying that the cable you’re using can support that resolution. It’s possible that the cable doesn’t have the bandwidth to support full resolution at the display’s refresh rate and is recommending downscaling in lieu of a lower refresh rate.