Ya gotta put your your fingers on a bullet to load it in a magazine.
And the bullet in the chamber is likely the last one you handled, or the second to the last.
That last bullet going in to a magazine, and even the others, are often pushed directly down, and in to position in that magazine, with the thumb.
Assuming that bullet was ejected, unfired, LE may have more than ejector/extractor marks. I wonder if that bullet could have found it's way there in some other fashion
It would be REALLY interesting if somebody else's thumbprint was on that live round they found.
I wonder if RA had range time, either at an actual firing range, or on the rural property of a friend, where a friend may have picked up one of his pre-loaded, cycled, bullets. There's several circumstances where a guy/gal would clear the chamber of a firearm, ejecting a live, unfired, round.