Because a module has many different impact points. If a missile barrage is targeted at a capital's side, it will hit differently than if it was targeted at the front. A missile barrage will not always hit the exact same place. Anyways, you were asking if there is a way to change the distance between impacts, it is based on the target's mesh and orientation. Missiles will hit farther apart on a broadside than against the bow.
Or, if by spread you mean the amount of missiles that are fired, then you can edit the number of shots per burst, the time between bursts, and the delay when swapping emitter points all in the ship's entity. The number of emitters is a mesh issue, but you already knew that.
If you want help with the above, then I'd love to help. Personally I'd go for a lot of emitters, a burst of one, and a large damage and delay between bursts to give that "OMGWTF RUN!" effect.
Too bad missiles can't swerve in flight.