Why does df show different space usage between two nfs volumes that have the same volume size when served by NAS such as NetApp filer?

Issue

• I'm looking at the following two volumes, '/symfoDB/sjdb1' exported from 'NAS1:/vol/sjdb1' and '/symfoDB/sjdb2' exported from 'NAS1:/vol/sjdb2':

$ df
Filesystem           1K-blocks     Used Available Use% Mounted on
/dev/sda2              5160576    373172   4525260   8% /
tmpfs                 16245352         0  16245352   0% /dev/shm
/dev/sda1               516040     41304    448524   9% /boot
/dev/sda9              2064208     72028   1887324   4% /home
/dev/sda7             12901520   1425712  10820448  12% /opt
/dev/sda10             2580272     69920   2379280   3% /tmp
/dev/sda8              5160576   3361036   1537396  69% /usr
/dev/sda5             12901520   1134000  11112160  10% /var
/dev/sda6             41284928  39034064    153712 100% /RDB
NAS1:/vol/sjdb1     223975872 216078208   7897664  97% /symfoDB/sjdb1 <<<<<
NAS1:/vol/sjdb2     223975872 223970432      5440 100% /symfoDB/sjdb2 <<<<<
NAS1:/vol/sjdb3     223975872 215982720   7993152  97% /symfoDB/sjdb3
NAS1:/vol/sjdb4     223975872 215982720   7993152  97% /symfoDB/sjdb4
NAS1:/vol/sjdb5      12582912  10532608   2050304  84% /symfoDB/sjdb5

• I see ls shows each volume has only one file of the same size as shown below:

$ ls -ls /symfoDB/sjdb1
Total 215886620
215886620 -rw-rw-rw- 1 root root 220200960000  Aug 31 22:04 2015 sjdb1

$ ls -ls /symfoDB/sjdb2
Total 215886620
215886620 -rw-rw-rw- 1 root root 220200960000  Aug 31 22:04 2015 sjdb2

• I see du also shows each volume has only one file of the same size as shown below:

$ du -ab /symfoDB/sjdb1
220200960000    /symfoDB/sjdb1/sjdb1

$ du -ab --apparent-size /symfoDB/sjdb1
220200960000    /symfoDB/sjdb1/sjdb1

$ du -ab /symfoDB/sjdb2
220200960000    /symfoDB/sjdb2/sjdb2

$ du -ab --apparent-size /symfoDB/sjdb2
220200960000    /symfoDB/sjdb2/sjdb2

• Why do I see the difference in space usage though each volume/file has exactly the same size?