I think it's because of the way the unit works. The carrier itself doesn't actually attack anything. It produces and spawns interceptors which are independent

I think it's because of the way the unit works. The carrier itself doesn't actually attack anything. It produces and spawns interceptors which are independent and targetable(but not capable of taking orders) units, which then go out and attack the carrier's target. The unit, by itself, is closer to a building than an actual unit, because the player has to manually queue up interceptor production, which has a resource cost and takes time. Since it does spawn the interceptors to attack, there are a lot of behaviors that it shows that other units don't. For one, the maximum range to a target where it can start to attack is fixed, like most units. At that range or lower, the carrier will launch it's interceptors, which will attack the target. However, there is also a longer "leash" range, where the carrier can move away from the target, after the interceptors have been launched, and they will keep attacking the target until the carrier goes out of the range, at which point the interceptors will go back to the carrier that spawned it. An explanation of this behavior can be found here: http://www.youtube.com/watch?v=1Rqx8s2qKXM http://www.youtube.com/watch?v=1Rqx8s2qKXM Also, when the carrier dies, all interceptors that it has launched die.