“200 shekels.” Although scholars differ about the weight, most agree it is about 5 pounds. This seems like an exaggeration, thus the addition of “according to the king’s weight,” which speaks to the accuracy of the weight.
“according to the king’s weight.” God wanted fair business dealings in buying and selling (Lev. 19:35; Deut. 25:13-16), and this required that weights and measures be standardized. It was the job of the Levites to maintain accurate weights and measures throughout the Israelite kingdom, but due to weights and measures being hand-produced and affected by use, accurate measures were hard to maintain throughout the kingdom. But the “king’s weight” would be accurate, and the phrase was added to assure people that the weight of Absalom’s hair was not an exaggeration.
That addition of the information about Absalom’s hair is important because it adds to credibility that Absalom could get caught in a tree by his hair when fighting David (2 Sam. 18:9).