The cun was part of a larger system, and represented one-tenth of a chi ("Chinese foot"). In time the lengths were standardized, although to different values in different jurisdictions. (See chi (unit) for details).
A section of an old Hong Kong ruler, showing the last (10th) cun of a chi. One can see that the chi in that jurisdiction was exactly equal to 14 and 5/8 of an inch. A metric ruler is shown next to it for comparison