In 2 Samuel 24:9 the numbers are different. The explanation there given is not so generally accepted as the supposition that the numbers have, in one passage or the other (or possibly in both), suffered corruption.
Related Commentaries of 1 Chronicles 21:5
1 Chronicles 21:5
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.