A person with an actual score of 80% will probably have enough confidence in his or her abilities due to experience that they will tend not to rate themselves low. Imagine being a graduate student asked how high (s)he would rank. They would not rank themselves as low as they might have when they were sophomore students. They would probably rank within 20% of their actual score, which is what the final graph in the article shows; professors have enough experience to be able to self-assess themselves better than less experienced subjects can.