The Mekhilta asks yet another question about the verse "And if one curses his father and his mother." From (Leviticus 20:9), which says "every man who curses," we would know only that adult men are liable. But what about women, or individuals of indeterminate sex?
This is where the Exodus formulation becomes essential. "If one curses his father and his mother" — the word "one" does not specify gender. From this inclusive language, the rabbis derived that the law applies to women as well as men, and also to a tumtum — a person whose biological sex is physically indeterminate — and a hermaphrodite.
The legal question is not theoretical. In a world without modern medical understanding, individuals of ambiguous sex existed and needed to be accounted for in the legal system. The rabbis did not simply ignore these edge cases. They systematically worked through every category of person to determine who fell under each commandment's scope.
The Mekhilta's method is consistent: whenever the Torah provides two parallel formulations of the same law, the broader one extends the narrower one. The Leviticus verse said "every man," limiting the penalty to adult males. The Exodus verse said "one" — any person — expanding the penalty to encompass all human beings regardless of sex or gender ambiguity. Together, the two verses produce a universal rule that leaves no category of person unaccounted for.