If Joseph Smith was reciting the Book of Mormon from memory, why would he be surprised by the content?

While dictating the Book of Mormon, Joseph asked his wife Emma (who was scribe at the time) if Jerusalem had walls around it. This occurred after he translated 1 Nephi 4:4:

Now when I had spoken these words, they were yet wroth, and did still continue to murmur; nevertheless they did follow me up until we came without the walls of Jerusalem.

Emma recalled this incident as follows (as reported by Edmund C. Briggs):
[O]ne time while he was translating he stopped suddenly, pale as a sheet, and said, “Emma, did Jerusalem have walls around it?” When I answered “Yes,” he replied “Oh! I was afraid I had been deceived.” He had such limited knowledge of history at that time that he did not even know that Jerusalem was surrounded by walls.
– Edmund C. Briggs, “A Visit to Nauvoo in 1856,” Journal of History, Jan. 1916, p. 454.

Why would Joseph ask this to Emma? Doesn’t this demonstrate that Joseph had not memorized the text in advance? If Joseph didn’t know Jerusalem had walls, how was he so proficient in other topics required to create the Book of Mormon?

See:

Elder Russell M. Nelson – A Treasured Testament

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