Because they can. Another example that does not involve a layoff. Years ago I was a very high performer but I changed organizations in Q4. Big mistake. Old org was not happy I was leaving so they kicked me out the door with a low rating which of course I did not find out about until February. Lesson learned, never change jobs (or announce you are leaving) close to year-end review time. Even if you let them know in late November or December upper management still has time to lower the rating and give some of your $$$ to others who are not leaving. They want to motivate people who will still be there not people that they know are leaving. Giving a high rating to someone who is leaving they consider a waste of bonus dollars when part of it could be given to others in the department, your peers for example.They view the bonus not as a reward for the prior year, but a motivator for the current year. Believe me. Also the How is a wild card that is difficult to dispute. That is why it was created.