LCDs have a limited viewing angle. As you approach the "maximum viewing angle", the display will darken. Continue to go further and it will keep darkening, eventually leading to color reversal. Most commercial panels are intended for dual use in TVs and in monitors. These panels tend to be optimized with a greater viewing angle going down than up so that putting the panel on a shelf across the room will give the best viewing experience. Unfortunately, most monitors are used from slightly above and at relatively close range, leading to some potentially unpleasant artifacts, especially if you're sensitive to color or intensity variance.

Another possibility is that the cold-cathode fluorescent lights used as backlights in many screens aren't perfect. The backlight gets reflected around from partly-silvered mirrors until the illumination field is "uniform enough". If the backlight gets bumped out of place slightly then the optics won't be quite aligned and you can get gradients in intensity. Another common defect is corners being darker due to non-uniform illumination from the backlight. LED monitors tend not to have this problem (or not as badly), but they are more expensive than CCFL backlit monitors.

Finally, some monitors have specs that allow visible variance between individual units. We have about 75 Samsung low-end 24" monitors at work that are marginal at best and show fairly wide variance in terms of overall illumination and viewing angle. They look fine individually, but when we put the same signal into all of them you can really see the differences.