Query For Blob Space

explain the difference in numbers between the queries? I am acutally more concerned about the ETM_XML clob since the descrepancy appears to be bigger.

SELECT table_name, column_name, segment_name, a.bytes FROM dba_segments a JOIN dba_lobs b USING (owner, segment_name) WHERE b.table_name = 'ETM_RAW_XML';

ETM_RAW_XML
IFD_XML
SYS_LOB0007260522C00012$$ 87870668800

ETM_RAW_XML
ETM_XML
SYS_LOB0007260522C00011$$ 125199974400

SQL> SELECT NVL((SUM(DBMS_LOB.GETLENGTH(IFD_XML))),0) AS BYTES FROM ETM_RAW_XML;
BYTES
----------------
83848300852 ~ 78.0898154266179 GB

SELECT NVL((SUM(DBMS_LOB.GETLENGTH(IFD_XML))),0) AS BYTES FROM ETM_RAW_XML;
BYTES
----------------
61907953222 ~ 57.6562743838876 GB