There is a difference in the account here given, from that of 2 Samuel 24:9. Perhaps Levi and Benjamin might not be numbered in the one as in the other.
Related Commentaries of 1 Chronicles 21:5
1 Chronicles 21:5-6
5 And Joab gave the sum of the number of the people unto David. And all they of Israel were a thousand thousand and an hundred thousand men that drew sword: and Judah was four hundred threescore and ten thousand men that drew sword.
6 But Levi and Benjamin counted he not among them: for the king's word was abominable to Joab.