Why is a small amount of unsaponified oil left in some finished soaps?

In the manufacture of bath soap, a small amount of unsaponified oil or free fatty acid is intentionally left in or added to the final product as a "superfatting" agent.1

This practice is primarily a safety measure to ensure that there is no excess alkali remaining in the finished soap, which could be harsh or irritating to the skin. By maintaining a slight surplus of fatty acids or oils—such as olive oil or coconut oil—manufacturers ensure the final product is gentle and safe for personal use.1