Radak: This is like "ka'Mishpat ha'Banos Ya'aseh Lah" (Shemos 21:9), i.e. the law and custom. Like Your rule and conduct to turn to those who love Your name, so do to me!

Malbim: Mishpat is what one deserves according to law. Chaninah is a free gift. Hashem has wondrous Hashgachah on those who love His name - "Raglei Chasidav Yishmor" (Shmuel I, 2:9), "Lo Ye'uneh la'Tzadik Kol Aven" (Mishlei 12:21). This is due to Mishpat; one who loves Hashem, he is beloved. David requested, even if he is not at this level, and he is not worthy amidst Mishpat, Hashem should give to him amidst Chaninah what He gives to Ohavav amidst Mishpat.