Why Furutaka began with six single gunhouses

Why was Furutaka originally built with six single main-gun mounts?

The design used the single mounts available at the time, although a four-twin-turret arrangement is presented as the better alternative.

Furutaka was built with six single main-gun positions because those were the mounts available to the original design. The resulting concentration forward resembled the arrangement in Nelson and Rodney and had awkward firing arcs. Four twin turrets are offered as a more flexible alternative, distributing the battery around the hull. Reconstruction therefore changed the geometry of how the cruiser could fight as well as its nominal calibre.