Why Britain settled on the 9.2-inch gun

Why did Britain use 9.2-inch guns rather than 9- or 9.5-inch weapons?

The calibre offered more useful shell volume than 9 inches while avoiding more of the wear and firing-rate penalties associated with approaching 10 inches.

Moving closer to 10 inches brought battleship-like costs in barrel wear and rate of fire. The smaller step from 9 to 9.2 inches nevertheless created appreciably more internal volume for shell shaping and explosive filling. The calibre was a deliberate balance rather than an arbitrary imperial measurement.