The root cause of all the variations out there now is Department size and economics. Nothing else.
Back when we all carried revolvers, the Department bought revolvers for every new recruit and issued them. When the Beretta 92 became the new LAPD issue, they bought enough to outfit each new recruit, but no way were they going to buy 8,000 more to equip all the veterans. So they made the Beretta optional for veteran officers. So now, there were revolvers and Berettas out there. When the LAPD authorized the .45 S&W, no way were they going to buy 9,000 more to equip all the officers who trained under Berettas and revolvers, so then they had revolvers, Berettas, and S&W semi-autos out there. When LAPD went to Glock, no way were they going to buy Glocks for everyone, so you get the picture, now we have revolvers, Berettas, S&Ws, Glocks, and now 1911s out there.... I am sure if the Department could enforce a single weapon policy, they would. But they just don't feel like buying 10,000 new weapons, and sending all the officers to a transition school.