Albert Barnes Commentary 2 Samuel 23:24

"Asahel the brother of Joab was one of the thirty; Elhanan the son of Dodo of Beth-lehem," — 2 Samuel 23:24 (ASV)

The early death of Asahel (2 Samuel 2:32) would make it very likely that his place in the Thirty would be filled, thus easily accounting for the 31 names in the list. Compare with the list throughout 1 Chronicles 11.