Albert Barnes Commentary 1 Chronicles 21:5

"And Joab gave up the sum of the numbering of the people unto David. And all they of Israel were a thousand thousand and a hundred thousand men that drew sword: and Judah was four hundred threescore and ten thousand men that drew sword." — 1 Chronicles 21:5 (ASV)

In 2 Samuel 24:9, the numbers are different. The explanation given there is not as widely accepted as the theory that the numbers have been corrupted in one passage or the other (or possibly in both).