Interceptors get depleted because the shot aren't actually coming in sequentially, all the fire is happening at roughly the same time. Your overwhelming the interceptors in capacity within a unit of time rather than endurance over the course of a battle.
You only have so many barrels and they can only maneuver around so much at once. Think of it like a martial arts movie. The guy who can catch arrows runs into trouble past two arrows at once due to only having two hands, not that the hands can't be used again in the next scene.
And yes I know, long winded explination of something you already know, but just for completeness sake...=)
Ripple