Grammar From Hell

If the tag line for the Deliver Us From Evil poster used “New York Police Department” instead of “NYPD,” the copy would read “Inspired By The Actual Accounts Of A New York Police Department Sergeant.” So why does this version say “…An NYPD Sergeant”? Am I missing something? Writing and grammar are what I do for a living, and this isn’t right. On top of which I don’t particularly want to see or know about Scott Derrickson and Jerry Bruckheimer‘s horror film, which opens on 7.2.