How far the rear derailleur moves with each indexed shift depends on a combination of two variables, cable pull and derailleur throw ratio. The "throw ratio" is a function of the geometry built into the derailleur. Because of these variables Campy shifters don't mix well with Shimano rear derailleurs. And pre-9-speed Dura-Ace shifters and RD's don't mix well with other Shimano shifters and RD's. And 8-speed and early 9-speed Campy shifters and RD's don't mix in with later Campy shifters and RD's.
(edit joejack951 beat me to it.)
Al