Let me explain it using a PC. Say you have a virus on a windows computer and you edit the registry to stop it from starting up when you boot up, if you shut it down and that programs still running in the memory, it can re-add itself to the registry - when you close an application it can perform certain tasks such as word; if you close it all the autosave files will be removed, the same process will happen if windows closes it down on shutdown. It is highly unlikely that you have/had a virus but it may be because of a program you used that may have been writing to that portion of the registry and stopped half way and/or you uninstalled that application. Not sure of the specifics really but it's definately an application, maybe a registry program but more likely to be a home plugin