Bullet collisions

I just started playing around with Robocode a few days ago, and pitted some robots against each other. I noticed that sometimes the bullets collide and then explode. Then I put two "FireTracker" robots, which are stationary and just fire directly at each other, into the arena with the expectation that they would just create fireballs between them as their streams of bullets collide. This didn't happen; the robots shot each other to death.

Now I'm confused...why do the bullets some times explode, but in the situation I created above they didn't?