Not that I'm defending the loss of these guns, but DHS is a huge agency, and includes ICE, Customs and Border Patrol, the Secret Service, the Coast Guard and other components that have a heck of a lot of guns. The agency has nearly 200,000 employees (though most admittedly do not pack heat).
Again, it's never acceptable, but human beings make mistakes; it would be interesting to see how these numbers compare to the number of losses among other law enforcement agencies, the DoD, and private owners.