In Linux/Unix world file deletion does not necessarily immediately free the disk space, it merely unlinks the file from file system.
If there are processes having file handles open to file being removed (as is common with syslog daemon, and in MySQL case, that mysql-slow.log), the space is not freed until the holding process is asked to reopen the file handles (HUP, hangup signal), or the process is restarted.
If you don't know what process is actually holding the file open, use
lsof -n | grep filename or if you want to see what marked-as-deleted-but-still-in-use files are present in system,
lsof -n | grep deleted