(Exodus 21:3) states: "If he were the husband of a woman, his wife shall go out with him." The Mekhilta asks: what kind of woman is this verse talking about? It must be a Jewish woman. But perhaps a gentile woman is intended?
The proof that a Jewish woman is meant comes from the very next verse. (Exodus 21:4) says: "If his master gives him a woman" — and the rabbis established that this refers to a Canaanite bondswoman. Since the master-given woman in verse 4 is already identified as Canaanite, the wife mentioned in verse 3 must be someone different. She is a Jewish wife whom the bondsman married before entering servitude.
The logic is clean: two consecutive verses, each mentioning a woman, cannot both refer to the same category. One is a pre-existing Jewish wife. The other is a Canaanite bondswoman provided by the master. The distinction preserves the coherence of the passage.
This matters because the two women have completely different legal fates. The Jewish wife goes free with her husband when his six years are up. The Canaanite bondswoman and any children she bore remain with the master. By identifying which verse refers to which woman, the Mekhilta established the legal framework that determined who walked to freedom and who stayed behind when the bondsman's term of service ended.