Our performance assessment process is like being part of a group that has to run away from fast zombies every year. It doesn't matter how fast you ran last year. It doesn't matter how fast you objectively run. The only thing that matters is not being one of the last ones in the current group because the last ones get eaten.
Every year, you have to hope that you are still not going to be the last one in the group. If you get promoted because you were among the fastest in the last group, you are probably now on the slow end of the new group. So be careful.
In a more sane system, it would be more like running away from classic slow zombies: as long as you run faster than some objective standard, you survive.
I wish someone had explained the system to me in these terms.