I consider that those sections of the pod that shrivel and blacken have either no seeds in them or only one.
After the flower is successfully pollinated the pod starts to develop. However, even though the pollination was successful it does not necessarily mean that seeds will develop successfully. The pollination may start development of the pod but development may then fail at later stages, such as fertilization of the ovules. Development of the seeds does not necessarily happen in all three chambers of the pod.
