Need Clarification about DB Storage

Hi ,

   I am just confused about the total database space usage.
   On my Oracle Enterprise Manager DB console home page, I see the Database Size (GB) : 535.97
   But when I goto tablespaces tab in OEM, I found Total_Size (MB):1,091,214.0  Total_Used (MB):938,198.0  Total_Free (MB): 153,016.0
   Why OEM db console space summary section on home page shows:535.97GB?

   If I run the below commands, I get the following results:
 SELECT SUM(bytes)/1024/1024/1024 AS Space(GB) FROM dba_data_files;

        Space(GB)
        -------------       
        982.963897705078

 SELECT SUM(bytes)/1024/1024/1024 AS Space(GB) FROM dba_segments;
 
        Space(GB)
        -------------       
        915.1826171875

   We have the above datafiles on ASM and if I run the following command: I get the below output: