Seventh. Syriac and Arabic add, "Eliu, (chap. xxvii. 18.) and the eighth David. "It appears, in effect, the Isai had eight sons, and that David was the youngest, 1 Kings xvi. 10., and xvii. 12. The Rabbins suppose that one was only adopted, namely, Jonathan, the son of Samma, noted for his prudence and valour, 2 Kings xiii. 3., and xxi. 21. (Calmet)
One might die in his youth, and be therefore omitted, (Tirinus) as he had done nothing memorable. (Du Hamel)
The Scripture does not always specify the full number. (Abulensis)
