Why a huge battleship might still carry nine guns

Why keep nine 16-inch guns on an 85,000-ton battleship instead of adding a fourth triple turret?

Because apparent deck space does not account for the turret's weight, magazines, armor, machinery, fuel, and competing systems.

Visible deck length does not prove that a fourth triple turret fits within the design. The mounting needs structural support, armored barbettes, magazines, ammunition handling, and crew spaces. Protection, machinery, fuel, secondary weapons, and other systems already consume the ship's displacement and volume. Adding the turret could therefore force unacceptable sacrifices elsewhere. Nine 16-inch guns can remain rational even on an 85,000-ton hull that appears to have spare deck space.