Caused by ambient light reflecting of the surface of the screen which is an artifact in your image. Take any direct view CRT, turn it off, sit in front of it and you will see the reflections of your room on the screen. There are anti glare screens but they add their own frosted looking artifact or discolor the gray scale on clear ones.
Best solution is to setup your viewing environment to reduce or eliminate this artifact on your screen.
see Screen Pitch