If coverage is a significant component in the performance difference observed between the hand-crafted and treebank-induced resources, then it is reasonable to expect that the performance difference is more pronounced with increasing sentence length (with shorter sentences being simpler and more likely to be within the coverage of the hand-crafted grammars).
we expect the hand-crafted, deep, precision grammars to do better on shorter sentences (more likely to be within their coverage), whereas the treebank-induced grammars should show better performance on longer strings (less likely to be within the coverage of the hand-crafted grammars).