You hear this being said throughout the saga. This leads me to the theory that Darth Vader wanted Like to survive and escape because he knew that Luke was his son and he wanted to turn him to the dark side. So I think the Stormtroopers are so good that they miss on purpose to carry out Vader's wishes. I think that they are really feared around the galaxy and are truly masters of marksmanship. Or if you want to put it simply, plot armor.
