Bullets don't always create shadows

I see robots creating a bullet shadow every time a bullet passes a wave but there is a chance that it doesn't create a shadow. For example, when two TrackFire fires at each other, the bullets don't collide. Probably I didn't understand the strategy but I just wanted to learn.

Sorry, I think I understood it. Bullets don't collide by their positions. They collide by the line(?) between it's current position and old position.