sum dbms_lob.getlength vs tablespace size

Hi helpers,

i have a tablespace which holds documents like pdf documents.

there is a blob column which holds the binary document.

When i use this select, then the sum of all blob columns is 12GB

select sum(dbms_lob.getlength(dokument)/1024/1024/1024), sum(lengthb(dokument)/1024/1024/1024) from dokumente

When i see the tablespace size in Enterprise Manager i see only 22MB ???

Why is the difference of the sum of blob column and the size of the tablespace so high ?

Another query dirctly on the dba_tables view with the blocks column shows 16,5 MB !!

select table_name,blocks*8/1024 from dba_tables where tablespace_name='DOKUMENTE'