simple physics
Ok, lets think about this. If you are driving along at 60mph and pass a stationary car it looks like you past it quickly. If you pass one doing 50 then it looks slow if you are going at the same speed then you never pass
If one ship was doing say 400kps and the other was doing nothing then ok, it would be impossible to target it. As it is all the ships are doing comparible speeds and are therefore not moving that much in relation to each other. In effect everything is almost done on the spot. the only guide to any kind of movement would be a static object, nav point or planet.
This is exactly how a fighter jet doing mach 2 does not hit it's own missile as it launces because the missle move in relation to jet.
Just a thought