In the Book of Mormon, Alma chapters 45 through 62 were written by Alma’s son, Helaman. Why isn’t that section called I Helaman, and the Book of Helaman, written by Helaman’s son, Helaman, called II Helaman, instead of being included in Alma?
Lehi authored 71 verses in First and Second Nephi, and they came to us as they were inscribed by Nephi on the Small Plates of Nephi, so why aren’t those verses called the Book of Lehi?.
From the Book of Mosiah on, the record is an abridgement of the Large Plates of Nephi, except for the brief writings of Mormon and his son, Moroni. So Mormon is the author of the record of Alma and Helaman as translated by Joseph Smith. In each of the books of the abridgement, Mormon records the records of various authors whose writings were recorded on the Plates that he abridged.
For instance, Mosiah 1-6 speaks of King Benjamin. The very words of King Benjamin are recorded in Mosiah 2:9 through Mosiah 5. We then have the account of Ammon and his dealings with the people of King Limhi and his successors in the Book of Mosiah, comprising chapters 7 through 16. Then there is the account of Alma the Younger from chapter 17 to chapter 27. Only chapters 27 through 29 deal specifically with Mosiah himself. So the Book of Mosiah could have been divided into four different books.
And so it is with all the books of the Book of Mormon. However, the record was divided into the different books of the Book of Mormon by Mormon himself, at his election, except for the books of Ether and Moroni. So to answer the question that you ask, you will have to wait until you meet up with Mormon, and then you can ask him.