sounds like a bag somehow ended up in the wrong container and when someone added up the bag count in each container v the expected total number of bags there was a discrepancy. I think there were issues with the baggage system recently so if more than usual is being done manually then mistakes can happen, especially if there are multiple flights to the same destination, hence the need for reconciliation.

or maybe as simple as a PAX never made it and his bags did

I have been on a (non BA) flight from BCN where the bags were removed from the hold out onto the tarmac and we all had to get off the plane and identify our bag before it was reloaded 2 flights and bags had been badly mixed up and ended up on the wrong plane