Employee performance MEASUREMENT appears to be Gaussian distributed. To my first simple, and let's be real probably somewhat wrong, approximation there are roughly 3 things that go into it.

1. There is a certain skill in communicating all the important things you've done, we shall lump likability + politicking into this one for convenience.

2. There is a premium that is placed on shiny new features and saving the day heroics. A lot less priority is placed on refactoring and solving the problems before they require heroics.

3. Finally there are individual's technical and self-management skills. I.E. it's important to work on important things and be good at it.