It is not until we add in the possible alignments that the percentage of four-word or longer phrases grows to 24% in the MSR RTE test set; when we look only at sentence pairs containing a least one possible alignment, the percentage of longer phrases grows to 44%.
evaluating only on the 406 sentence pairs that contain at least one possible requires systems not only to perform well on longer alignments, but also to avoid sacrificing performance on short alignments.