Charles Ellicott Commentary 1 Chronicles 21:6

"But Levi and Benjamin counted he not among them; for the king`s word was abominable to Joab." — 1 Chronicles 21:6 (ASV)

But Levi ... —This verse is missing in Samuel, but it probably existed in the original source. There is nothing in the style to suggest a later author; while the word “counted” (pâqad), which has not been used before in this chapter, occurs twice in the parallel passage (2 Samuel 24:2; 2 Samuel 24:4). It is noticeable also that the chronicler writes “the king” (not “David”) here, as in Samuel.

Regarding the fact stated, we may observe that the sacerdotal tribe of Levi would naturally be exempted from a census taken for military or political purposes. (Numbers 1:49.) And 1 Chronicles 27:24 expressly asserts that the census was not completed, a result with which Joab’s disapproval of the scheme may have had much to do. The order in which the tribes were numbered (2 Samuel 24:4–8; see 1 Chronicles 21:4) makes it likely that Judah and Benjamin were to have been taken last, and that, after numbering Judah, Joab went to the capital, where he was ordered by the king to stop the undertaking. Josephus (Antiq. vii. 13, 1) speaks as if Joab had not had time to include Benjamin in the census. He may have feared to offend the tribe of Saul.