After googling around I found the issue:
The /tmp
folder has the sticky bit special permission:
drwxrwxrwt 11 root root 36864 Sep 23 14:12 tmp
Note the t at the end of permissions - this means that the sticky bit is set. When the sticky bit is set, any user can create files, but only the owner of a file can delete it.labor-liber.org
From Ubuntu Support:
Within a directory upon which the sticky bit is applied, users are prevented from deleting or renaming any files that they do not personally own.