My experience is that the politics and BS lead to BS ratings that go against the written policies because there are contradictory backroom policies.
In this market, we can't seem to hire anyone (we aren't competitive on salary). So I guess it makes sense that they want to keep the low performers since they wouldn't be able to backfill. This still sucks since there's a lot of stress dealing with poor ratings and not know if you will get fired. And then even if you do work hard and perform well, there'd no guarantee that will increase your rating or comp.
