Derivations generated for these documents are such that both translation model features (with or without the LM) and retrieval features agree on a path close to the SMT Viterbi translation.
other relevant documents require more non-standard lexical choices that are harder to achieve in a +LM search space, since the strong weight on the language model, plus a language model-driven pruning technique, strongly favor lexical choices that agree with the language model's concept of fluency.