I'm more interested in why employees don't band together to game the system and ensure a given outcome, then split the winnings. Like a sort of a pyramid scheme - you and a buddy agree that this quarter it will be you who gets promoted and split the bonus/raise with your buddy. Then you get a third buddy and get him in on it if he agrees to let the second buddy be promoted this quarter. Now you're splitting 3 raises…
Also, that sounds like a horrible amount of concerns for a developer. I'd like to build and ship code, not worry about measuring performance.