Rulers are less accurate with the curvature of the body.
Holding a standard ruler over the wound is less accurate when it’s not in the same plain as the wound. At 4 cm, the difference is 0.5 cm.
The standard ruler would imply that the wound is smaller than reality.
The KISS Ring with its adhesive backing scales in the same plain as the wound. Its area scale is a single unit that does not change scale with curvature. The area inside the green ring is 25 sq cm. It helps keep the perspective of the wound intact.