Why the British calibre became 9.2 inches rather than 9 inches

Why did navies use a 9.2-inch calibre instead of simply calling it a 9-inch gun?

Because the bore was roughly 233-to-235 mm, which converts more closely to about 9.2 inches than to 9 inches.

Nominal inch labels often approximate dimensions expressed more naturally in millimetres. A bore around 233 to 235 mm converts to a little over 9.2 inches and can approach 9.25 depending on the exact weapon and rounding convention. Calling it a nine-inch gun would understate the calibre by several millimetres. Calling it 9.25 inches would imply a precision that the family of weapons did not consistently share. The familiar 9.2-inch name is therefore a practical rounded label produced by moving between measurement systems.