 Because you can see in the original assassination attempt video that President Trump shifts the angle of his head at the last moment to look at a chart that they put on a big screen. And in doing so, he shifts the angle of his head so the bullet travels through his ear as opposed to through the back of his head. According to the Wall Street Journal, investigators found rudimentary explosive devices in the shooter's car parked near the rally in Butler, Pennsylvania. They also found bomb-making materials at the family home one hour to the south. Again, the FBI says that he acted alone.