Stormtroopers seem to operate by Law of Conservation of Ninjutsu rules, in that the more of them there are, the worse they are, as the amount of accuracy and competence is divided amongst all the Stormtroopers present.
Maybe the electromagnetic fields generated by their blasters energy bolts repel each other, so that when a half dozen troopers are aiming at the same target their blasts are subtly pushing each other away and making each other miss. (Meanwhile, Imperial QA testing is firing them one at a time, when they are perfectly accurate, and passing the blame on to the Stormtroopers, since they are engineers, and Imperial engineers always blame the user for product deficits.)