That's not a whole lot of ammo. The DHS employes tens of thousands of law enforcement/military members (USSS, Coast Guard, Customs/Border, FPS, etc.)
Between the Coast Guard (40,000 active duty + 40,000 reserve/aux) and Customs/Border (45,000 active duty) patrol and FPS (1,200 active duty), and USSS (4,400 active duty), you're talking 130,000 law enforcement/military personnel. 750,000,000 rounds equates to around 5-6,000 rounds/person (and the above doesn't include all the law/military enforcement officers the DHS employs).
That's not an unreasonable amount of ammo for training for around one year. I burn through at least a couple thousand rounds a year just for recreation.
Also, the DHS runs the Federal Law Enforcement Training Center, which does training (including lots of firearms training) for around 90 agencies (federal, state, local, etc.). Include all the people training there and they probably burn though that much ammo in months -- just for training.