Some times when you delete a file df does not show the true file size (i.e. it does not consider the deleted file size ). This is due to the fact that some processes still have a handle on the file i.e. they might still be using the file. This can be confirmed by the following command
as root run "lsof -n | grep file
to see the list of processes still having a handle on file.
You may kill the processes or restart the system to fix this.