Debras gun in season 6 episode 1

Rewatching and I've just got to the first episode of season 6 when Debra takes down the guy in the bar before she get's made lieutenant. I tried to count the shots she made with her Glock 17 and it sounded/looked like 15 which might make sense as a G17 mag holds 15 rounds, but watching back in slow motion I can see she only made 14 shots before she was empty. What happened to the other two rounds? (assuming a police detective would also have one chambered) Is Debra a secret serial killer herself and she double tapped someone before going to meet Quinn at the bar?