Brant Gardner writes that in Mosiah 11, the first person writing style of Zeniff's account is now replaced with a clearly synoptic third person account. One might ask, Why did Mormon make this shift in editorial method and why did he make the change here? The answer is another subtle proof of the veracity of Mormon's text.
To begin with, the original account of Noah is likely to have been long. As evidenced from Mormon's report of Noah's public building projects and his excess show of wealth, Noah was a self-absorbed man. Such a man would make sure his official record was impressive. Noah's only problem, however, was that Mormon was apparently not impressed with such a prejudiced record. In fact, Mormon's introduction to Noah begins unflatteringly, which is certain to be in contrast to the tone of Noah's official record. But more importantly, Mormon shifts to telling us about Noah in a third-person synoptic style because Noah's original records would not have revealed the real Noah. [Brant Gardner, "Book of Mormon Commentary," [http://www.highfiber.com/] ~nahualli/LDStopics/Mosiah/Mosiah11.htm, p. 1]