There was an earlier report that the magazine was still full, and that only the chambered round was fired. Presumably this detail was released by the police—only one bullet was fired.
Several people commented that maybe the reporter got the story wrong, since ordinarily a round from the magazine would have replaced the one that was fired in the chamber.
But, it now looks like that earlier report was correct, and that Mas Ayoob has given a likely explanation for it.
The press are the same people who think evil semi-auto "assault rifles" spray bullets like a Tommy Gun.