Albert Barnes 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 - The twelfth according to 2 Kings 8:25. The discrepancy may be best explained by the two ways of reckoning the accession of Ahaziah, who was likely regent for his father for at least one year. See 2 Chronicles 21:19.