Khya texted me today.
(I doubt he'll ever blag on his own, so I've taken him on as a contributor to my blag. Jaja yes!)
So I had another etymological epiphany: EXACT. Ex-meaning "out of," and Act-as in "what takes place." So when used nowaday, we mean it so to say "that which comes directly from the action/source." So an "exact copy" is a mirror image because it comes out of the act, the source.