Why does it say "Al Peshatetem"?
Rashi, Targum Yonasan: The Lamed is in place of a Nun; they are interchangeable, e.g. "La'asos Lo Nishkah" (Nechemyah 13:7) is like Lishkah.
Radak citing R. Yonah: It is if as the Alpeh in "Al" is an Ayen, and the word "Mi" is omitted (whom did you despoil today)?
Radak citing Ibn Ezra, Malbim: Did you not despoil today? (David would answer where they despoiled.)
Did David say that he raided all these places every day?
Radak: Sometimes he would tell him southern Yehudah, sometimes southern Yerachme'eli?
Who are Yerachme'eli and Keni?
Radak: Yerachme'eli is a family from Yehudah, from Yerachme'el ben Chetzon (Divrei ha'Yamim 1 2:9). It is mentioned also when David sent to him from the spoils (30:29) - "vela'Asher b'Arei ha'Yerachme'eli vela'Asher b'Arei ha'Keini." Keini are from Yisro's family; they converted and lived among Yehudah, like it says in Shoftim (1:16).