Space really used on TBS

Hi,
executing the following query

SELECT b.tablespace_name, a.*,
       TRUNC (a.segment_size / tbs_size * 100, 2) segment_perc_used,
       b.tbs_freespace,
       TRUNC (b.tbs_freespace / c.tbs_size * 100, 2) freespace_perc,
       c.tbs_size
  FROM (SELECT SUM (BYTES) / 1024 / 1024 segment_size
          FROM dba_extents
         WHERE tablespace_name = 'USER_CDC_DATA') a,
       (SELECT   tablespace_name, SUM (BYTES) / 1024 / 1024
tbs_freespace
            FROM dba_free_space
           WHERE tablespace_name = 'USER_CDC_DATA'
        GROUP BY tablespace_name) b,
       (SELECT   tablespace_name, SUM (BYTES) / 1024 / 1024 tbs_size
            FROM dba_data_files
           WHERE tablespace_name = 'USER_CDC_DATA'
        GROUP BY tablespace_name) c

I get this result

USER_CDC_DATA,112,0.72,5076.8125,32.75,15500

where 112Mb are used (0.72% of tbs total available space)
5076Mb are free (32.75% of tbs total available space)
15500Mb is the tbs size

Why I have less then 1% used and only 32.75% free?
Where I'm wasting space?
Please note that shrink objects on tbs doesn't provide any benefit.

Regards
Mauro