Maybe a person is in a bad role, on a bad team, or simply doesn't have experience or skills fitting the specific requirements of that single point in time. I've been a high performer in the past. Now I'm a low performer (I'm slow) after the company has changed the way it treats teams/work and the technology shifted. My disability isn't suited to the new working style/process (or lack of). However, I do excellent work outside of my regular day to day responsibilities, like my secondary role as an ASC. My manager even said that I'd I could just speed up my regular work I would easily get the highest rating and a promotion. Hearing him say that really sucks because I unable to just speed up with my disability in the given work environment. The company/managers have made no effort in helping me find a role that works better with my skills or is impacted less by my disability. It seems there may not be any of the traditional roles/teams left. So now I fill midlevel roles and move team to team trying to find somewhere that I fit in. If I can find a place I fit in like before, then I would easily excel.
Another example was a dev on another team that I worked with a few years ago was sloppy, slow, and just all around seemed like a poor performer - and even our tech lead and manager talked poorly about him. He was even considerably slower than me. About 3 years after that he's now the head of data and analytics for our international operations.