In addition to the headers at the beginnings of named books, Mormon will at times add a header for a chapter. These chapter headers typically reference a source that is not the large plates from which Mormon is taking most of his record. We saw several of these in the book of Alma, where Mormon was using a separate, personal record that Alma had created.
This chapter header is similar in that Nephi, the son of Helaman, is the Nephite recordkeeper, but there was apparently a separate record that he kept. Perhaps it is the same separate record from which Mormon will take most of 3 Nephi, but Mormon does not make that clear.
There are two unusual features in this chapter header. The first is that it appears to be given a name: “The Prophecy of Nephi, the Son of Helaman.” The second is that it includes a reference to the record of Samuel the Lamanite, whose prophecy will have its own chapter header (before Helaman 13). In the case of the prophecy of Samuel, it may indicate that it was recorded on Nephi’s personal record rather than the large plates, and that Nephi gave it that header. However, it still could be Mormon because he gives chapter headers to the records of the sons of Mosiah, whose records appear to have been copied into Alma’s personal record.
It is also possible that the interpretation of “The Prophecy of Nephi” as a title is simply a misunderstanding by the compositor. There is a prophecy that is important, but the whole of this record cannot be considered a prophecy. Therefore, it is best to see “the Prophecy of Nephi” as simply one of the descriptions of the contents rather than a title.