The reason is that unless there is a range restriction against a certain kind of ammo, shooting your duty carry ammo is preferred. When I go to the range I am using my daily duty carry ammo out of my daily carry magazines to ensure it functions properly. Some ranges have restrictions on lead, so some frangible ammo is purchased for those facilities.
I work for a small agency in DHS that supports BP and ICE and we typically shoot at a minimum 1000 rounds a year, if we only get to the range once a quarter. That is 600 rounds for basic quarterly qualification and about 400 rounds of low light, transition, immediate action, and movement drills. If you think about it, that isn't that much.
Within DHS, CBP has something like 50,000 officers that have to shoot quarterly at a minimum. While many of the port inspectors are only shooting one gun, BP agents might be shooting as many as 4 for qualification. I don't know where the big buy for FLETC Glynco is going. It seems like a lot, but it can get burned up pretty fast depending on what training programs it is going to.
Good post