I think the 'administrative immigration charges' means that those 1,855 didn't have prior criminal histories.
Apparently a new term. I looked it up on google and the only two close hits were this article you posted.
Calpernia's right...illegal entry is a misdemeanor, and the administrative charges are what get filed.
The second entry, I believe, is a felony. They're supposed to throw the book at you then. Yeah, right.