"Then they shall sell the living ox" — when one person's ox kills another person's ox, the Torah prescribes a specific remedy. But the Mekhilta specifies: this verse assumes the two oxen were of equal value. What if they were not?
The Mekhilta addresses the inequality with a logical observation. What is the normal outcome for the person who caused damage? He loses. His position deteriorates. And what is the normal outcome for the person who was damaged? He recovers the amount of his actual loss — no more. The damaged party does not profit from the incident.
If we assumed oxen of unequal value, problematic scenarios would arise. A cheap ox might kill an expensive one, and the simple remedy of selling the living ox and splitting the proceeds would leave the damaged party far short of his actual loss. Conversely, an expensive ox might kill a cheap one, and the damaged party might end up with more than he lost.
To avoid these distortions, the Mekhilta reads the verse as establishing its rule for the baseline case: oxen of equal value. In that scenario, selling the living ox and dividing the proceeds — along with dividing the value of the carcass — produces a fair result. Both parties share the loss equally. This baseline case teaches the principle, while cases of unequal value require more complex calculations that the Mekhilta develops elsewhere in the legal tradition.