why Ext3 filesystem shows used space that doesn't match with available space in DF command output?

Issue

• Ext filesystem shows used space that doesn't match available space in df command output .

Filesystem                     Size  Used  Avail  Use%  Mounted on  
/dev/mapper/vg00-lv_dump       63G   58G   924M   99%   /dump

• The issue is the available space is 63 GB and used is 58 GB but df shows 99% filesystem is full.
• If disk size is 772GB and 73MB is used then why df shows available size 733GB?