etc. The early death of Asahel 2 Samuel 2:32 would make it very likely that his place in the 30 would be filled up, and so easily account for the number 31 in the list. Compare throughout the list in 1 Chronicles 11.
Related Commentaries of 2 Samuel 23:24
2 Samuel 23:24
24 Asahel the brother of Joab was one of the thirty; Elhanan the son of Dodo of Bethlehem,