Why do du and df values not agree when other mount points are present?

Issue

The file system with issues is /opt/oracle which is ext3 filesystem.

[root@system]$ df -h /opt/oracle
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg10-lvoraopt
                      5.0G  4.8G     0 100% /opt/oracle

[root@system]$ cat /etc/fstab|grep -i /opt/oracle
/dev/vg10/lvoraopt      /opt/oracle             ext3    defaults        0 0

• Below, we see the files under /opt/oracle consuming approx 650M.
• The df -h shows its utilized 100% i.e. 4.8G.

• This happened suddenly and no data was copied to the mount point before mounting the filesystem.

• The directories which are mounted on separate filesystem are marked (Separate File System)

$ du -sh /opt/oracle/*
12M     admin
5.7G    CRS (Separate File System) 
7.1G    GRID (Separate File System) 
4.0K    lost+found
11M     ocfs2 
4.0K    oracle_software
7.4G    oracrf (Separate File System) 
8.8M    oradiag_oracle
1.3M    oraInventory
271M    orcl (Separate File System)  
271M    orclMirror1 (Separate File System) 
10M     orclMirror2 (Separate File System) 
12K     PD1
12K     PDC
588M    product
4.0K    sqlnet.log