When you delete a file, you simply remove its entry from the hard disk index. In other words, you never erase a file, but only its reference.
Once deleted, your files can be recovered using tools such as Photorec or TestDisk.
To get rid of a file really you'll need to use secure deletion tools available on Windows, Linux, Android and Mac OS. Unfortunately not yet on iPhone.
The "Empty Recycle Bin in Secure Mode" function has been removed from OS X 10.11. Apple felt that it could not guarantee secure deletion on SSD drives.
If you're using a traditional hard disk with OS X 10.11 and can use the command line, you can use the srm command to overwrite the file.
If you know how to use the terminal on Mac OS, since Mac OS 10.12 you can use the command
rm -P