I was always curious myself, but this solves it.
Aside: Nobody really seems all that accurate with the Stormtrooper rifles. I mean, in the prison escape scene, as well as later when Luke and Leia are on the unextended bridge. I think maybe those particular rifles just suck or something. But most likely its a dramatic element. Like others have said, you cant kill all the main characters off in their first encounter with the ultra-precise stormtroopers.