If self evaluations are random, and you group a bunch of them together, then you'll see values around the 50th percentile. That's why their self evaluation line

If self evaluations are random, and you group a bunch of them together, then you'll see values around the 50th percentile. That's why their self evaluation line is nearly flat. In the actual data though, the line clearly trends upward. The people who did well appear to be scoring themselves non-randomly.