Something isn’t quite right. They shouldn’t be in his line of sight from his seat on the box.
He wasn't sitting on the box while firing any of the three shots.
Watch
The Lost Bullet, again, for cryin' out loud.
I'll find out how many minutes into the video for you to start watching, and post it here for you, okay?
-- MWT
Edit: The segment in The Lost Bullet where they analyze the sniper's positions, how the spent casings ended up where they did, etc., etc., starts at 43:56 ...
Google "the lost bullet" video