Or, simply said, from the Engine's point of view, our second is 16 ticks game-time...and time_step is the duration of the last frame (but not in milliseconds, in ticks!, so around 0.267 average on 60fps (16 ticks/60 frames)). By decrementing with 0.267 every frame, you're decrementing with 16 points per second!
If you want to decrement with 1 per second, divide time_step / 16, but keep in mind that small numbers can become inaccurate.