Those are (usually) totally different.
Your 'Original' is your
what was first imported
The 'Original_*' is what (of the same format) things started with.
eg
You start (original) with Mobi
You convert to EPUB (still no 'Original_*')
You
Polish the Epub. Presto! Original_Epub'
There are other same-format actions that will create the Original_*
There are Tweaks (save_original_format) to control which happen
BTW I use the words 'Master Format' to describe the true original. I also archive this outside of Calibre,
just in case 