Unskilled and Unaware of it

Alternatively, those who performed best on the tests could simply be lying about their perceived abilities. They know full well their grammar / logical reasoning / whatever is up in the higher reaches, but knock a few points off when declaring their perceived abilties in a partly subconcious effort to avoid looking like an arrogant know-it-all. Nobody likes a braggart, and the better you are at anything the more self-effacing you can afford to be about it.