Apple uses what's popularly known as a "panel lottery," that is, they use panels from different companies for the same computer. For example, the Retina MBP comes with either a Samsung or LG panel. So different iMacs may look different.
I must admit I'm not an expert on display calibration. If you've played with the built-in calibration utility for a while and can't get what you're looking for, either you'd need professional hardware/software to do it, or it may not be possible. (They'd never look exactly the same unless they were the same panel.)