The space for any library can be overallocated. This excess space can be released manually with the FREE command in ISPF. Or you could code the release (RLSE) parameter on your JCL or select a management class that includes the release option partial.

RLSE is complemented by the partial release parameter in the management class, which can cause DFSMShsm to release free space during the daily space management cycle or depend on the equivalent in the management class. This function works the same for PDSEs as it does for PDSs or sequential data sets. The partial release parameter frees the unused space but it does not consolidate extents.