Jesus told the people in Bountiful that he had not specifically told those in Jerusalem about them. What he did was mention that he had other sheep. Now, he tells those in Bountiful the very same thing. Jesus Christ is the Savior of the World, and neither those in Jerusalem nor those in Bountiful should consider themselves unique. There are yet other sheep, and those other sheep must still hear his voice.
We do not know who those other sheep were, nor how they heard his voice. What we do know is that if the Nephites had not kept their record, we would not have known that he came to them. Thus, if Christ visited others and they did not write it, or their records did not survive time, we would not know of that visit, even though it had been made.