The difference between the total capacity of a disk/volume and the space occupied by files is available for new files and thus considered to be free. However, most operating systems allow files to continue to exist after they have been deleted if they are still accessed by applications. Those files do not show up when calculating disk usage (e.g. with
du on Linux), but also do not show when looking at free disk space (e.g. with
df on Linux).
The disk space occupied by these files is called _leaked_. On Linux you can find the files leaking disk space by calling
lsof | grep deleted.
Leaked disk space is usually not a problem, especially log files that have been rotated can cause this. However, if leaked disk space continues to increase, this indicates a problem, usually originated by application code not closing file handles.