In the real world, when two ships would be flying at the same speed, no matter the direction, a shot fired from one of them would fly with the same relative speed to either of them, no matter how fast they are speeding to or away from a planet.
SC however takes a different approach - shots are fired with fixed velocity - the podship for instance fires his plasmoid with a very low speed, and a druuge mauler fires his shot with high velocity. If the theory of relativity would apply, the plasmoid would be fired with the start speed plus ship speed. Since this is not the case, many mycon capitans were forced to regrow half of their crew after being hit with their own plasmoid - when the podship is speeding faster than the start speed of the plasmoid.
So, to give an example - grab a mauler, and start fireing up. When you do gain some serious speed. notice that when you shoot furhter up, the shots have 'bigger range', and when down, the range is even shorter than a Yehat Terminator. Because of this, when an enemy is chasing you, your shots 'gain' speed, that is, just as they would gain the speed of the opponent.
If the X would be a cute little Pkunk, then it would be on a colison course with the shot. If there was no planet, it could look like the shot is flying with double speed at the horrified Pkunk. Since you could say that the shot is faster (although it's really the pkunk speeding into the bullet), it will have bigger range before it dissapears.
In the real world the shot would be first decelerated or accelerated by the speed of the fireing craft.
Although it may not be real, it is fun