This is due to the fact that the denominator factor must be accumulated for all higher order states that do not have the given n-gram.
table 5 presents perplexity and WER results for less heavily pruned models, where the pruning thresholds were set to yield approximately 1.5 million n-grams (4 times more than the previous models); and another set at around 5 million n-grams, as well as the full, unpruned models.