I cann't give you a definative answer, but I know that the Monarch scheduled services operate under a Zb prefix and the charters as MON.
I suspect therefore that responsibility for the flights is taken by 2 seperate bodies. Monarch themselves control the inventory on the ZB flight, but a charterer (Tour operator) is responisble for the other service.
I recall flying from Manchester on the charter service and had been told by Avro reservations (a Monarch company) that it should be possible to change to another flight. When I got to the ticket desk I was told in no uncertain terms that this was not possible.
If you can remember back to the days of British Airtours, I am sure that you would not expect British Airways to accept a British Airtours ticket?! The same parent, but trading as seperate companies.
Regards
Scottie Dog