Cause #1 is sequential, turn-based model of combat. One way or another, one opponent has a free turn to punch enemy and only survivied ships can fire back. Illustration: you and enemy have 10 identical ships that can kill say 5 ships in one round. Let's assume it is a warp-poit battle (ships are in range) and you fire first. 5 enemy ships are dead. 5 remain, fire on you and kill 2.5 (2 dead, 1 damaged) ships. Next turn, you finish enemy. Result: two _absolutely_ identical fleets, identical race bonuses, etc., but you kill them all with only 20% loses.
If only we could make it more or less simultaneous...
Ship initiative ala Moo2, anyone ?