It's likely that you are seeing a discrepancy in the disk space accounting, where it does not accurately track actual disk usage. Bugs in the space accounting component have been found and fixed from time to time. A particularly significant fix (DR 82822) was included in the V6R2.0 release. If you are using an earlier version, you may want to consider upgrading.
Space accounting discrepancies are repaired automatically when the DBS restarts, as all disk usage is recomputed at that time. You can acomplish the repair without doing a restart by using the UPDATESPACE utility. See the Utilities Manual for details.
It depends on what you consider significant. It could cause some queries to fail unnecessarily on a bogus "out of spool space" condition. My advice is to use UPDATESPACE to repair the accounting for the affected databases and then continue to check periodically for recurrences.