Why Can't Bad Guys Shoot Straight?

Here's a still from 'Cowboys & Aliens' that serves as yet another example of The Principle of Evil Marksmanship aka The Stormtrooper Effect.

So... These aliens have advanced technology, can travel vast distances, possibly utilizing some kind of time-warp drive, find our planet in an obscure solar system in a nondescript galaxy on the other side of the universe, but they can't hit a horse at full gallop?

You know, you look at the still, they're not even close. They've missed the nearest rider by -- looks to me -- about 50 to 75 feet.

Why can't bad guys in movies shoot straight? Here's a clip from a documentary that sheds light on the subject:



That explains a lot:






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