If our hypothesis is correct we expect the "grammar higher" group to have more accurate parses than the trigram-higher group as the poor parse would cause poor grammar perplexity for the sentence, which would then be worse than the trigram perplexity.
secondly, as noted in [2], the parser encourages right branching with a "bonus" multiplicative factor of 1.2 for constituents that end at the right boundary of the sentence, and a penalty of 0.8 for those that do not.