Difference Between 'du' And 'df' Commands

'Df' reports space allocated to file blocks, while 'du' reports space used by files in the directory tree. In normal circumstances these are the same of course (apart from rounding errors, which should be small), but if a file is deleted while some process has it open it disappears from the directory structure but its blocks remain allocated. This is the normal cause of df/du discrepencies.

'Lsof' is a very useful tool to help find the deleted file(s). You have a large discrepency so look for big ones.