Matthew tightly couples verses 14–15. The city on the hill rephrases the same concept as the candle on the stick; the conjunction “neither” signals their relationship. The 3 Nephi text breaks that link, altering verse 15 so completely that a declarative sentence must be recast as a question. The meaning is not significantly altered, but the flow of the text is. The city on the hill becomes an isolated phrase, floating separately from its context, while Matthew used it to introduce its parallel, the candle. In the process of clarifying the English wording, Joseph inadvertently uncoupled the concepts.
There might be some who would suggest that such an imperfection in the process must necessarily deny the process. That is, if Joseph made a mistake, he therefore couldn’t have translated by the power of God, since God does not make mistakes. I refuse to buy into that “logic.” This passage reveals Joseph at work, telling us that the Lord used a human being, weaknesses and all, to further a great work. Bringing forth the Book of Mormon involved Joseph Smith the person. That is the method. Evidence of how it was done does nothing to diminish the miracle of what was done.