Q. I saw that the Sefer HaChinuch divides Mishpatim into two separate Sidrot, each with its own count of mitzvos. He calls the second one “Im Kesef”. Why?

A. In his introduction to Sefer Shemos, Abarbanel explains that Sefer Bereshis contains twelve parshios that describe the life and achievements of Adam, Noach, the Avos and Yosef and his brothers. Since Moshe Rabbenu is the equivalent of all of them, Sefer Shemos also contains twelve sidros. Since, as any Chumash will testify, we only maintain eleven parshios on Sefer Shemos, it must be that the Abarbanel also agrees with the Chinuch and Rabbenu Bechaye that divide Mishpotim into two distinct parshios.
Maharid (on the Chinuch ”“ Mishpotim) further explains that the addition of Parshas Im Kesef was necessary to be able to read Parshas Metzora right before Pesach during a leap year. Thus complying with the remez or allusion to the siman “saggru upasschu” (close and open). Saggru being Metzora that requires confinement. Biur Halocho (428: 4) adds that Parshas Metzora contains the instruction of breaking earthenware utensils, similar to the kashering done before Pesach. (See also Orchos Chaim ”“ Krias Hatorah 1, 3). Maharid adds, that reading Mishpotim as two parshios is still the tradition of the communities of Algiers and Tunis. See also a similar division at the end o Abudraham Hasholem (p. 374).
It is interesting to note that Machzor Vitri (at end, p. 808) mentions that Parshios Tetzave, Ki Tissa and Pekudei, on a leap year would also be divided into two, giving rise to fifteen parshios in Sefer Shemos. (Haeshel p. 27).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a