Charles Ellicott Commentary 2 Kings 9:29

"And in the eleventh year of Joram the son of Ahab began Ahaziah to reign over Judah." — 2 Kings 9:29 (ASV)

In the eleventh year of Joram. — 2 Kings 8:25 says in the twelfth year of Joram. Such a difference is not remarkable, since the synchronisms between the reigns of the two kingdoms are not based upon exact records. Moreover, different computations might make the same year the eleventh or twelfth of Joram. (The verse is a parenthesis, and perhaps spurious.)