Windows 10 and Monitor Resolutions - Virtual and Physical Mode Change
This change was made to reduce flicker when changing between resolutions, but because the resolution sent to the monitor does not change, your monitor image may be letterboxed.
On virtual resolution compatible hardware* running Windows 10 when you change the resolution through Display settings you will notice that whilst the resolution has changed, the monitor OSD still states that it is receiving a video signal at its native resolution:
This is because Windows will scale the image to the new resolution instead of physically changing the resolution.
To force a physical mode change, you either need to edit the monitor advanced settings, or logoff, or reboot your machine.
*Compatible hardware means a PC/notebook/tablet with WDDM 2.0 compatible graphics drivers.