A: Ring mandrels are typically engraved with the sizes and then hardened. The hardening process causes the steel to expand and shrink along the mandrel, so that the engraved sizes are no longer 100% accurate. Unfortunately, the engraving cannot occur after the hardening process. And if the ring mandrel is not hardened, any hammering that occurs on it will change the shape and size of the mandrel as well.
To deal with this problem, it is always best to have a finger sizer along with a ring mandrel and check the sizes on your ring mandrel with your finger sizer for comparison.