Superiority bias
One problem is the ambiguity of the word "average". It is logically possible for nearly all of the set to be above the mean if the distribution of abilities is highly skewed. For example, the mean number of human legs is slightly lower than two, because of a small number of people have only one or no legs. Hence experiments usually compare subjects to the median of the peer group, since by definition it is impossible for a majority to exceed the median.