TSM server V5.1.6.2 on z/OS.

Can someone explain why these numbers from a Q DB F=D don't "add up" (at
least they don't make sense to me ?):

          Available Space (MB): 35,160
        Assigned Capacity (MB): 35,160
        Maximum Extension (MB): 0
        Maximum Reduction (MB): 5,372
             Page Size (bytes): 4,096
            Total Usable Pages: 9,000,960
                    Used Pages: 4,968,239
                      Pct Util: 55.2
                 Max. Pct Util: 55.2
              Physical Volumes: 15

In summary, it says that of the roughly 35GB DB, only 55% is used.  In
round numbers, 20GB is in use. This means that 15GB is not.

However, the "Maximum Reduction" value is 5GB.

Where is the other 10GB ?

Even if TSM kept the "reduction" values based on the volume sizes (roughly
2.3GB per 3390-3 volume is what it is reporting), I should still be able
to reduce the size by 6-volumes, or roughly 13GB !