create_table_cost

 I am trying to use create_table_cost procedure in oracle 10.2.0.3. For one of my tables I am getting the used bytes and allocated bytes values as 986546176 and 986644480. Close enough. But when I check the sum(bytes) from dba_segments for this table I get a value that is about two times higher - 1983610880. Is there a reason for such a discrepancy and how do I interpret it - what is the 'real' size of this table?

thank you