Of course this statement could be perfectly accurate, but it has the feel of a moral. The statement that there were more converts than those that died has the feel of an extrapolated moral. The righteous sacrifice of the people resulted in an even greater conversion. In the context of the moral structuring of this story, this particular detail has the feel of contrived example rather than actual history. It could have been very true. It could also be an artifact of Mormon’s editorial process.
Seeing Mormon as an active editor who is reworking his material for a particular purpose should not diminish his efforts in our eyes. Mormon is an editor in the ancient mold, not a historian in modern sensibilities. While the modern historian might value the ability to suggest “what really happened,” (while still being subjective in how that was constructed), the ancient author was driven more by his purpose that his data. The gospels in the New Testament are excellent examples of where the editorial purpose of the writer flavored the way “facts” were presented, including times when the “facts” were perhaps less than factual. For instance, the Matthean genealogy is clearly manipulated to fit into three groups of fourteen generations (Alan Hugh M’Neile. The Gospel According to St. Matthew. London, Macmillan & Co LTD, 1961, p. 5). Mormon is simply a historian in the ancient mold, where the purpose of the story is more important that the absolute accuracy of some of the facts. The purpose of the facts was to enhance the story.