Thats because every iMac display is individually colour-calibrated using state-of-the-art spectroradiometers to match colour standards recognised around the world.
http://www.apple.com/uk/imac/features/
Was this always the case with iMacs, or is this a new feature, as a photographer if this is a new edition over the 2011, its alot more useful to me than most the upgrades and will bring the screen closer to a dedicated calibrated screen.
Still not sure about 2011 maxed out or 2012, disgusted an SSD is not standard in top end 27", fusion drive should of been 128gb minimum spec for the 27" with 256, 512 uprades, not stuck with a 1tb HDD....