Without a doubt, these words are pregnant with meaning, and I will not be able to capture all that is going on here succinctly—but I will give a basic treatment. The words in question is/are: יָקָר. Yaqar (lambs) and כַּר kar (fat); (or כַּרר kar) can also mean pasture where lambs feed (see Davidson 394); and כָּרִים (karym) which can be rendered as “’vine dresser’; well-cultivated plain, garden, orchard, field” (Davidson 393); or vineyard (see Botterweck 8: 319). In Hebrew, like English, some words can carry a variety nuances or meanings.
Also, when read in the Hebrew we have the sonic, poetic quality of alliteration and pun and in context a simile כִּיקַר כָּרִים (ky-kar karym); “as the fat of lambs” or “glory of the pastures.” cf New International Version renders this as “beauty of the fields” (again accurate given the nuances of the Hebrew words)—a similar expression chosen by NAS scholars.
In translation of the Bible from Hebrew and Greek, translators often make choices of either a “word for word” translation or “dynamic equivalent.” In this case both the KJV and NAS scholars are most likely using an expression that captures the dynamic equivalent of the Hebrew for their time periods respectively—that is to say, an expression (or figure of speech) that is both true to the Hebrew and most easily understood by the modern audience for which it is translated.
That being said, the expression “Fat of the land” can be found in modern literature such as in John Steinbeck’s Mice and Men (1937)—(meaning the richness of the land) which is of course a biblical allusion from the KJV translation as found in Ge 45 “And take your father and your households, and come unto me: and I will give you the good of the land of Egypt, and ye shall eat the fat of the land.” Note the expression “fat of the land” is similar to “fat of the lamb” (KJV).
The NAS (published in 1960) chooses an expression that is equally true to the Hebrew (“glory of the pasture”) but with a more modern vernacular. The expression “the glory of the pastures” (while accurate) just doesn’t have the same poetic register as “fat of the lambs”—but of course, that is a matter of opinion—and it is just as precise as “fat of the lamb.”
Additionally, in modern English, ‘fat’ has taken on more negative connotations than in Elizabethan (and other) times—being associated with “unhealthy”, etc. (as opposed to “richness”) therefore, modern translators might opt for more positively charged word (in this case ‘glory’); and “of the pastures” could be another way to say “lambs”—creatures who are closely associated with pastures—because that is where they feed (see Davidson).
Also, I am inclined to think the KJV scholars might have been having a little fun here because the expression is so close to “fat of the land” which is a similar expression found in Ge 45—of course I cannot prove that—put it registers as a bit “punny.”
In any case, the different renditions do not change the message of the text—the enemies of the Lord are going to be “vaporized” (will quickly perish) like a burnt offering or like “green pastures in the heat of summer” (Craigie 298).
Sources
Botterweck, G. Johannes, et. al. Theological Dictionary of the Old Testament (15 Volumes).
Wm. B. Eerdmans Publishing Company, 1995.
Craigie, Peter. Psalms 1-50 Word Biblical Commentary. Word Books, 1983.
Davidson, Benjamin. The Analytical Hebrew and Chaldee Lexicon. Hendrickson Publishers1995.
Dotan, Aron. Biblia Hebraica Leningradensia. Hendrickson Publishers, 2001.