Why a new gun standard left older weapons in service

Did attempts to create one standard gun simply add another standard to those already in service?

Yes. New guns entered service while older ships kept their weapons because development funding was not matched by money for retrofits.

A new gun could become an additional standard when older ships retained their existing weapons. The navy could receive money to develop and fit the new gun, then be refused the funds needed to retrofit earlier ships. Repeating that sequence accumulated ammunition and mounting requirements across the fleet. The answer also describes the difficulty of saving an annual allocation until research and a comprehensive refit program could be completed together.