Jesus had given the Israelites a single hint that some of his covenant people were not in Israel in the passage about “other sheep” (John 10:16). At this time, he quotes that reference to the New World twelve. I hypothesize that he did this so that the twelve, though instructed that they will remain separate, may be comforted by knowing that they were not forgotten while Jesus served his earthly ministry in Israel.
Translation: The entire section on “other sheep” relies on the New Testament context and cultural familiarity with sheepherding. Jesus is the good shepherd and brings his flock together. Because there were no sheep in the New World, Jesus must have communicated this information using culturally familiar imagery for his Nephite audience. Joseph, in his translation, reverted to language more familiar to his New Testament audience.