Try this. In your original example, slow the bullets to 200. You'll see what happens.
It's the same thing. The flames are drawing on top of each other as you go forward and farther apart as you go backwards.
Think of it. If you had a flame throw and sprayed in front of you. If you ran forward, you would get burned - the flames are still in the air in front of you. They don't move forward just because you did.