'Rashi': It is for the honor of David.

Chomas Anach (Shmuel II Perek 1, citing Get Pashut 129:121): A Yud was added to David's name (in this Sefer, and sometimes in other Kesuvim), and a Hei to Yonason's name, to hint that Hashem's name Yud-Kei was between them, like Chazal say (Sotah 17b, Rashi there) about a man and his wife. It is a woman's nature to marry a man. Yonason willingly added a Hei to his name, in order to be like a wife to David. David said (Shmuel II, 1:26) that his love was more than love of women.

'Rashi': Yes. Now the pearl (David) was found, so the Sefer does not mention the eighth son, Elihu. Radak and Malbim, citing Yalkut Shimoni 2, 147) - "Elihu me'Achei David" (27:18) was appointed Nagid over Yehudah. He was younger than David. David is called "ha'Katan", for he was Maktin (diminished, i.e. humbled) himself.

Radak citing the grammarian R. Yonah: It says in Shmuel I, 16:10-11 that Yishai passed his seven sons in front of Shmuel. Shmuel asked if there is another; he said yes - the Katan, David. If so, David is the eighth! We must say that "Shevi'i" means eighth ¹ .

Radak: One of the first seven who passed in front of Shmuel was from a different wife. Here it says that David was seventh from his mother. Also Ibn Ezra said so. Malbim - the coming verse lists their sisters, including Avigayil, whom a verse calls "Bas Nachash Achos Tzeruyah." Since she was from a different father (Nachash, not Yishai ² ), she was not a sister at all to David's half-brother. Therefore, here it lists only David's maternal brothers. Avigayil is Achos Tzeruyah, and not Achos David, for she was Tzeruyah's full sister, but only a half-sister to David. Their mother married Yishai after Nachash died.

Radak: Some say that one son died ³ .