When a file is written to a disk, it has a certain number of sectors or clusters allocated to it. The area of disk space provided, is always larger than the file itself. Deleting a file alone, leaves a space which can contain sensitive data. There are a number of ways in which this sensitive data can be deposited without a user knowing it. It is in the nature of a computer, to always be updating one file or another.
Applications can create huge numbers of such files.
When a file is eventually deleted, only the last image is accounted for. All other images appearing as free disk space, unseen, unsuspected. Much depends on their intended use. All versions will then remain available for deletion. Choose a deletion program with the ability to perform multiple overwrites.