Id almost bet you got full screen scaling enabled in your display driver.
NVIDIA User:
Right Click empty desktop -> Nvidia Control Panel -> Find resolution scaling and turn it off
AMD User:
Right Click empty desktop -> AMD Radeon Software -> Settings -> Display GPU Scaling off
Also make sure your internal resolution is 1080p at 100% scaling
EDIT:
If all of this fails:
Create a custom resolution (like 1920x1080p by hand) in your display driver and set it as internal resolution