When you look at the Work with System Status (WRKSYSSTS) display, you may not be getting all the information you think you're getting. Under the Auxiliary Storage statistics, the system storage amount and the percentage-used numbers only reflect the system ASP. If you have additional user ASPs, they will not be included in these figures. Under this scenario, it is possible to segment your memory into separate user ASPs and have WRKSYSSTS show less memory than before the segmentation.
Currently, the only way to show valid storage figures is to run the Work with Disk Status (WRKDSKSTS) command and add up the usage numbers by hand.
IBM made this design decision and incorporated it into OS/400; and, according to IBM, there are no plans to remedy the situation. Perhaps if we complain loudly and often, IBM will eventually change it.