The agency where I was working made the switch from issuing .40 S&W (Glock 22) to 9mm (Glock 17) a few years back. The primary reason given at the time was to streamline ammo and firearm purchases. There had always been a certain number of female or older officers who could shoot 9mm more accurately than .40 S&W. Basically, most officers could qualify with .40, but everyone could qualify with 9mm. The ballistics were considered similar enough to be negligible, but the chain of command never stated or implied that 9mm was superior.
Officers did like the additional magazine capacity, and the department liked that 9mm ammo was the more economical choice. I do not believe Admin would have made the change if they thought it would jeopardize officer safety, but budget issues will always be a major concern with any LE agency. In my experience, agencies don't adopt any firearm or ammo because it is "the best"; they adopt the equipment/technology that can safely and reliably accomplish the goal for the lowest price.