This may be regarded as favoring n-grams that are likely to appear in the reference translation (because they are likely in the derivation forest).
in order to score well on the BLEU metric for MT evaluation (Papineni et al., 2001), which gives partial credit, we would also like to favor lower-order ngrams that are likely to appear in the reference, even if this means picking some less-likely highorder n-grams.