While Nephi's purpose in writing is to tell a spiritual story, he does want to get the facts straight. This verse adds important information, but it is totally out of place chronologically. This is because the time in the wilderness was not part of Nephi's narrative, and he spoke as little of that time as he could get away with. The birth of his brothers was not event that moved along his moral story. Nevertheless, as he begins to tell the next significant spiritual occurrence, Nephi briefly stops to catch us up on an important fact. It is almost as though in the years when Nephi is writing, he comes to the departure, and realizes that his brothers entered the ship with him, and that he has neglected to mention them. He corrects that oversight, and moves on to the story that is the focus of his narrative.