Not a rule... just funnier this way! 😁

Also, the creator of the magic mirror trap quite possibly adjusted the mirror to slightly modify the doubles because 1) he's still harboring a grudge against our heroes, or 2) he's suppressing some fetishes that are coming out in the magic items he creates (might be hard to recognize, but that is a Cat 'O Nine Tails her double is holding in her left hand... 😏)