well watch the movies

Imperials did run around when shooting and missed alot, well they even missed when they where in a prone position (Episode IV --> troopers chasing Luke and Leia)

When chewie shoots, he stands most of the time still to aim his crossbow, and most of the time, they are direct hits

the trooper who hit Leia in episode VI stood perfectly still

And sometimes it was just luck when they killed a person when running (Han)