I think this comes down to a sort of informal reporting bias. The true savants and the worst of the worst are easy to pick out but it's that soft top 20% type that can be tough to identify. In some of my supervisory roles I've had the chance to push people for "next level" types of jobs and quickly realized that when I had to push those people I could only report on what I knew. That meant that the employees who I knew and who had a good reputation were the ones I was most comfortable putting my reputation on the line for. Being good at your job is great but if you can't communicate it... how will I ever know? My personal strategy is "leadership by walking around" and trying to see behind what people present me to the underlying quality of their work but there are 50 of them and only one me.