That reduces the double images that give rise to the stereo sensation. Smaller double images mean less crosstalk. Color selection helps too. White lines on a blue background produce little crosstalk. The image values are compressed, which unfortunately reduces contrast and makes everything a little pastel.
Crosstalk is greatest at the bottom of the screen because of phosphor afterglow.
One needs an especially light grey background there to counteract the strong crosstalk. One could have this light grey background uniform over the whole screen, making everything even more pastel. Or, toward the bottom of the screen one could have the grey background ramp up in brightness, which might be distracting. We chose a darker uniform background that allowed some crosstalk at the bottom of the screen. Images often have little there anyway.