Have a look at the title page. This page was written on the last leaf of the plates themselves and was not written by Joseph Smith. Rather, as with the record beginning with the Nephi and ending with Moroni, Joseph translated it to English from an ancient language.
The title page states that this book is an abridgment. It's not the comprehensive history of the people it talks about. The timeline it treats is between 600 BC and 420 AD. Instead, it is a comparatively brief summary of the high and low points and the main spiritual themes and issues of its day.
The author of this title page, Mormon, writes also that the purpose of this book is "the convincing of the Jew and Gentile that JESUS is the CHRIST, the ETERNAL GOD, manifesting himself unto all nations" (italics added).
From this, we learn that the purpose of the Book of Mormon and the purpose of the Bible are one and the same. Therefore, as we read, we should not be surprised that the teachings about Christ are also one and the same. In some cases, the words Christ spoke to teach the people of the Americas are verbatim with the Bible's account. (And why shouldn't they be? The Gospel is the same universally, regardless of who hears it, when, and where.)
