One reason is it's not the same display...in other words, not the same TFT unit as in the Cinema line.
iMac 17"
- 1440 by 900 resolution
- 350:1 contrast ratio
- 120° horizontal & 90° vertical viewing angles
- 200 cd/m brightness
- 'unstated' response time
- 'unstated' pitch
iMac 20"
- 1680 by 1050 resolution
- 350:1 contrast ration
- 170° horizontal & 170° vertical viewing angles
- 230 cd/m brightness.
- 'unstated' response time
- 'unstated' pitch
Cinema 20"
- 1680 by 1050 resolution
- 400:1 contrast ratio
- 170° horizontal & 170° vertical viewing angles
- 250 cd/m2 brightness
- 16ms response time
- 0.258 mm pitch