We believe that this is due to a mixture of the following: (1) 'capacity', i.e., model complexity, of the parsersthat is, risk of overfitting; and (2) few, but very long sequences on essay level-that is, little training data (trees), paired with a huge search space on each train/test instance, namely, the number of possible trees on n tokens.
there are 5338 relations, most of which are supporting relations (>90%).