I read the research paper mentioned in the article, "Unskilled and Unaware of It," by Kruger and Dunning and it was very good in explaining why average students think they should get better scores. It was one of those Lake Wobegon stories where all of the kids were above average. It was only after the students had improved their skills a bit and compared their new scores with the older score that they started to understand how poor they were in the first place. A simple example was that students who scored in the 12th percentile actually thought they were in the 60th percentile.
It also showed that the very best students underestimated their abilities and almost always thought that they could have done better.