There was one very short article about this, maybe three paragraphs or so, that got reposted on ten thousand sites around the web. And its very very hard to find any additional details- however, the contract is for "up to" that many, spread out over five years.
Here is a relatively raw (IE, not too opinionated) version of the story.
ATK Secures .40 Caliber Ammunition Contract with Department of Homeland Security, U.S. Immigration and Customs Enforcement (DHS, ICE) - MarketWatch
You will notice that only the first year is solid, the subsequent 4 years are referred to as "option years", and the minimum order is never mentioned, only the possible maximum.
So its very possible the the government is only really ordering 1 million, or ten million rounds. Who knows- you certainly cant tell from this press release.
My guess is the actual sale amount will never approach 450 million rounds.
As to why Homeland Security (which would include the FBI, and various other actual law enforcement agencies, not just the TSA or ICE) needs that much ammo, who knows? It aint target ammo, is it? Even if it was, I cant imagine all those agencies combined shoot that many rounds in five years at the range, and they probably only shoot a few thousand in actual working situations, annually.
It is fishy, just not nowhere near as fishy as the overly optimistic press release makes it seem.