It is probably more a matter of semantics. Artillerieoffizier usually denotes an officer in charge of low angle gunnery; Paul Schmalenbach was the Flak officer for Prinz Eugen, but as this was also the secondary armament, he had also the distinction of being the II AO. In Bismarck the heavy flak would not normally be employed in surface fire, so while Gellert was third in seniority it was more usual to designate him by his primary role as Flakleiter to avoid confusion (I suspect). Muellenheim-Rechberg, as fourth in the list of artillery officers was officially the IV AO as he was backup for Schneider and Albrecht.
Their shoulders held the sky suspended;
They stood and Earth's foundations stay;
What God abandoned these defended;
And saved the sum of things for pay.