Put me in the "commonality is over-rated crap" camp. We've been stuck with that mindset since 1994. Previously had an approved list (S&W, Beretta, Glock, Ruger...9mm or .45) that we could choose from. Then we had a horrendous gun battle with a heavily armed nutjob, and guys were trying to share mags without success......because we went through hundreds of rounds of pistol ammo shooting at a rifle armed suspect wearing body armor and hunkered down behind several cars against a building line. So of course....the problem was that guys couldn't share magazines! When some of us pointed out that the problem was that it was a rifle fight and we didn't have any....we were looked at as potentially dangerous mental patients. We are still stuck with that mindset, 22 years later.
As for agency procurement at the local level....it is usually about the preferences/fetishes of one, maybe two, people. Our agency has never sought input or tested with the folks who will actually carry the things and have to shoot them. It's about someones gun rag porn fetish.