Rockets and bombs have the second point: the ground.
Gun rounds have a lifespan, and will expire after 1.2, 1.5 whatever that lifespan is. Otherwise, you'd only see the green X on the ground, like the other LCGS. That's where all bullets in real life end up!
So, basically, you can't compute it in a 3D reference... It has to be in relation to something, meaning on a target plane or on the ground. Otherwise the green Xs are just floating in space at an arbitrary point in front of you, rather than in relation to where the rounds actually need to go.
I don't think I'm describing it very well, but I don't know how to word it.