Vilna Gaon, Malbim (from Yerushalmi Rosh Hashanah 1:1) #1: A large calculation swallows a small amount. (Months are not counted with a large number of years.)

Vilna Gaon (from Yerushalmi Rosh Hashanah 1:1) #2: The six months that David fled Avshalom [are not counted, for he did not have the law of a king. Had he sinned], he would have brought the Korban of a commoner.

Malbim (from Sanhedrin 107b): The six months that David was a Metzora are not counted.

Radak (Shmuel II, 5:4): The Klal (total number) omits the excess, for it is not a full year. In Melachim I, 2:11, it is omitted even in the Prat (his reign in Chevron alone).

Radak (Shmuel II, 5:4, from Yerushalmi Rosh Hashanah 1:1): The six months are lacking from the 33 years in Yerushalayim. For the honor of Yerushalayim, the half year there is counted like a full year.

Radak (Shmuel II, 5:4 citing Yerushalmi Rosh Hashanah 1:1: For six months, Yo'av and Yisrael fought Edom (based on David's command). Hashem said "Al Tisgaru Vam" (Devarim 2:5)! Therefore, they are not counted towards his reign.