This did not surprise me at all. “apple has been sued for its 20 inch lcd displays which has thousands of colors less than the announced specifications”.Though the 24-inch version has a standard LCD, the 20-inch iMacs have a 6-bit twisted nematic film (TN) LCD screens, which are cheaper than the standard LCD.
The 20-inch iMac's TN screens have a narrower viewing angle, less color depth, less color accuracy and are more susceptible to washout across the screen.
Though it is very difficult to find the difference between the two LCD's in normal use, But when it comes to image editing and high resolution photoshop work the differences are obvious.
Best part is that the new 20-inch monitor was a upgrade of previous version(which had a better and standard LCD 😉 ), actually customers paid more to get a inferior quality monitor(Maybe the upgrade was for Apple's profits).