This is trawling back into my dim-and distant, but I hope someone will have the facts to hand. If I remember it had something to do with the number and type of guns available at the time, between time the design was laid down and the time it came to arm her. Production rates meant that it was better to go ahead with the installation as known. Also, Bismarck/Tirpitz were a different class from Scharnhorst/Gneisenau, no such thing as a standard turret. Very, very similar between class vessels, but could still be significant difference in any navy between two capital ships.
"Well, my days of not taking you seriously are certainly coming to a middle." - Malcolm Reynolds