"Deflect" and "stop" are two very different actions. A mere twig will deflect a rifle bullet but won't stop it. It doesn't take much to deflect, or change the course of, a bullet
In Africa I had a twig deflect my .338 Swift A-Frame bullet, missing my trophy bull. The twig was close to me, if it were close to the target it wouldn't have mattered as much. If the twig were on the target it wouldn't have mattered period. Deflections happen all the time, especially so if you do not see the object in the way.