Stormtroopers were the clones, and cloning can get messy. Not to mention, there are theories about how the stormtroopers are actually good shooters, but they kept missing on purpose to let Luke and everyone get away so they could figure out where the rebel base is. Princess Leia even says it herself. At the beginning of the movie, they were very good, killing the ship crew, blasting Leia, but suddenly on the Death Star, it's all gone. Ben Kenobi says that Troopers are precise.
Now clones are bred to be shooters, while the FO Troopers were just babies, do they couldn't really measure how their aim would be. Now there are some with raw talent, others trained and became good, and some just aren't that great at shooting. So clones would be better.
Also, Luke could be using the force in some cases to slightly steer off their aim, make the blasts just miss. Far fetched, but it makes a little sense.