Negative value in table

Hi all,

   Db version 9.2.0.4 32 bit on Linux.

   I imported db ago three days. Today, I use script to view size of objects such as:

SELECT owner, segment_name, segment_type, partition_name, ROUND(bytes/(1024*1024),2) SIZE_MB, tablespace_name
FROM DBA_SEGMENTS
WHERE SEGMENT_TYPE IN ('TABLE', 'INDEX', 'TABLE PARTITION', 'INDEX PARTITION',
'TABLE SUBPARTITION', 'INDEX SUBPARTITION')
AND OWNER ='XBANK'
AND tablespace_name = 'XBANKDATA'
ORDER BY bytes DESC;   

    But some tables has negative value at bytes (it's SIZE_MB) column.  

XBANK

XBANK_DATA_1

TABLE

-40464046.4

XBANKDATA

   I don't know why it's negative value ?