there are some exception that may result on what you are experiencing like:
1- If you delete a file in some file managers or GUI environments, it may be put into a trash area where it can be undeleted.
As long as the file can be undeleted, its space is still consumed. and it will be calculated on the du -h command.
2- If the file is open by a process, the data remains. Only when the processes have closed the file is the file deleted.
The output fuser -m or lsof on a mount point includes the processes that have a file open on that filesystem, even if the file is deleted.