Space on unix file system and Disk Usage

We are approaching our db size limit in AIX 5.3.  When performing the unix command   ‘df -g’ it returns that we are using approximately 51 gb of memory out of our 64 gb allocated.  However, when performing the unix command ‘du -g’ the return states that we are only using a little over 11 gb.  Can anyone explain why these are different and which one is correct?

Thanks!