Your screen has a glass faceplate that reflects--into your eyes--light that originates behind you. Reflections can change your perception of your display at the location where it is reflected. The flatter your monitor's faceplate, the less of a problem reflections are likely to be; a highly curved screen "collects" reflections over a wide angle behind you.
To determine whether your screen has reflections, sit in your normal working position and examine the dark screen for reflections. (The reflections may be distorted by the curvature of the screen.) Try to arrange your environment so that no intense light sources are reflected on your screen. If you cannot move your furniture, either move the light source or block your (reflected) view of the offending object with dark cardboard baffles.
Your monitor's screen may have an integral antiglare coating or treatment to minimize glare. A monitor with this treatment appears to have a very dark screen when it is turned off. You can attach an external antiglare screen to the front of your monitor, but some antiglare screens have such low light transmission that you may find that they reduce the intensity of white to an unacceptably low level.