Why does the Book of Mormon seem to know that “let his face shine” is a Hebrew idiom for “smile”?

The priestly blessing of Aaron to the children of Israel, as recorded in Numbers 6:24–26 says:

The Lord bless thee, and keep thee: The Lord make his face shine upon thee, and be gracious unto thee: The Lord lift up his countenance upon thee, and give thee peace. (emphasis added)

This scripture is very similar to 3 Nephi 19:25 which says:

Jesus blessed them as they did pray unto him; and his countenance did smile upon them, and the light of his countenance did shine upon them. (emphasis added)

The key difference between the two is the word “shine” in the Bible, and “smile” in the Book of Mormon. Evidence Central explains:

The New American Bible has a footnote to Numbers 6:25 which indicates that the expression “let his face shine” is “a Hebrew idiom for ‘smile.’” Additionally, biblical scholar M. I. Gruber has explained that the phrase in verse 26, “lift up his countenance,” should be rendered idiomatically as “smile.” With this in mind, the statement that Jesus’ countenance “did smile” upon the people in 3 Nephi 19:25 is a very appropriate expression of the Hebrew idiom found in Numbers 6:25–26.
The English word smile doesn’t show up in the King James Version of the Bible. And yet here it is in 3 Nephi 19, being used in a way that appropriately expresses the underlying Hebrew idiom from Numbers 6:24–25, a passage which is clearly being alluded to, based on both wording and context. Not only is this allusion textually appropriate, but its presence strongly suggests that whoever authored it was familiar with the underlying Hebrew idiom in Numbers 6.
– Evidence Central – Book of Mormon Evidence: Christ’s Priestly Blessing

Where would Joseph have learned this? Why did he not ensure someone noticed these small details as evidence that the book was true?

See:

Evidence Central – Book of Mormon Evidence: Christ’s Priestly Blessing

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