This is most likely just accounting errors - which the Army does all the time. Even if it's not an error, it's then probably the Iraqis who lost them, not us.
The US Army is extremely careful about losing rifles & pistols. If soldiers on a training outing in the US lose a rifle, the entire unit has to go out into the field and search until they find it.
At a certain point you have to show trust in the Iraqi police and armed forces by letting them be accountable for their own inventory. Not sure if that "power" was transferred or not, but it appears to be a bigger screw up on the part of the Iraqi's and not so much on the US Army