That's yer problem. The 20" screen is a different animal from the 24" screen.
Read this link...
Eye On Apple Here is the pertinent part from the link...
"Apple told consumers that both the 20-inch and 24-inch iMacs displayed �millions of colors at all resolutions.� Indeed, the new 24-inch iMacs display 16,777,216 colors on 8-bit, in-plane switching (IPS) screens, as did the previous generation of 20-inch iMacs. But the new 20-inch iMac monitors do not even come close, displaying 98% fewer colors (262,144).
While Apple describes the display of both the 24-inch and 20-inch iMacs as though they were interchangeable, the monitors in each are of radically different technology. The 20-inch iMacs feature 6-bit twisted nematic film (TN) LCD screens, the least expensive of its type.
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.
Apple�s Web site tells consumers that �No matter what you like to do on your computer � watch movies, edit photos, play games, even just view a screen saver � it�s going to look stunning on an iMac.�
In fact, the inferior technology of the 20-inch iMac is particularly ill-suited to editing photographs because of the display�s limited color potential and the distorting effect of the color simulation processes."
