There may be some sound reasons that explain why the KJV may have a lower average syllable count than some present English Bibles such as the NKJV, and those reasons would have no direct bearing on whether or not it is actually easier to read and especially on whether or not it is easier to understand.
For example, in most editions of the KJV there are several commonly used words that are divided into two or more words where the exact same word united as one word in another English translation may count as a longer, multi-syllable word.
Some examples include “to day,” “to morrow,” “for ever,” “for evermore,” “son in law,” “father in law,” “mother in law,” “daughter in law,” “strong holds,“ “way side,” “high way,” “good will,” “any more,“ “any thing,“ “mean while,” “mean time,“ “some time,” “sea side,“ “sea shore,“ “mad man,” “free man,” and “cart wheel.” There may also be other such words. While later KJV editors changed or corrected a few of the uses of “lift” in the 1611 edition for the past tense “lifted,” there are other times where a present KJV has “lift” while the NKJV may have “lifted.” Sometimes the NKJV may have an adverb spelling which may add a syllable while the KJV has an adjective spelling used as an adverb [for example, “more frequently” in the NKJV for “more frequent” in the KJV]. The KJV may present numbers with more words with fewer syllables [for example “forty and three” in the KJV where the NKJV has “forty-three”]. A few words may be united in the KJV that are divided into two words in another translation. Overall, because several of those words divided in the KJV are much more commonly used words, they would contribute to giving the KJV a lower average syllable count. Those divided words do not actually make the KJV easier to read and easier to understand. By the way, some present KJV editions would unite some of those words such as “to day” to either “to-day” or “today” so that those KJV editions would have a different average syllable count. Many times the 1611 KJV edition had “shall be” united as one, longer word “shalbe,” and it would likely have a different average syllable count than a present KJV edition.
The KJV could perhaps have as many as several thousand uses of the one-syllable word “and” where the NKJV does not, which would also lower the KJV’s syllable count. This would not actually make the KJV easier to understand.
More importantly, the KJV has a number of archaic words or words used with archaic meanings that may be shorter or have fewer syllables than their present English equivalents. Some examples could include the following: “turtle” for “turtledove,” “vale“ for “valley,” “dearth“ for “famine,” “trump“ for “trumpet,” “tongue“ for “language,” “coasts” for “borders,” “host” for “army,” “wood” for “woods” or “forest,” “table” for “tablet,” “even“ for “evening,” “let” for “hinder,” “anon” for “immediately,” “oft“ for “often,” “sod” for “boiled,” “awaked” for “awakened,” “jeoparded” for “jeopardized,” “mete“ for “measure,” “dure“ for “endure,” “ware” for “aware,” “quick“ for “living“ or “alive,” “mean“ for “common,” “still” for “continually,” “attent“ for “attentive,” “by and by” for “immediately,” “ere“ for “before,” “minish” for “diminish,” “fine” for “refine,” “grave” for “engrave,” “astonied” for “astonished,“ “strange” for “foreign,” and “rid” for “deliver.” While such words may help reduce the KJV’s average syllable count, they do not actually make it easier to read and understand. The KJV's archaic language may help give the KJV a lower syllable count, but it does not actually make the KJV easier to understand. The archaic language in the KJV may mislead present-day English readers and make it harder to understand correctly.