What is the justification for distinguishing between joint liability in homicide (Nafs) versus joint liability in limb mutilation (Atraf)?

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Limbs require exact equality (tasawu) for retribution; thus, a healthy limb cannot be taken for a crippled one, nor a full set of fingers for a partial one, nor original for extra parts, nor right for left. This equality necessitates prohibiting Qisas among joint perpetrators concerning limbs. Equality is not required for life; a healthy person is taken for a sick one, and sound limbs for amputated or crippled ones. In limb cutting, equality in the act of cutting is required; if each cuts from a different side, Qisas is not due, unlike homicide where the pain affects the life irrespective of the specific location. Joint liability for homicide occurs frequently, necessitating Qisas as a deterrent against murder, whereas joint liability in limbs occurs rarely, lessening the need for such deterrence.