Because this camera is using PIR to detect motion, PIR is detecting the heat emit from heat source. A truck will emit more heat than people, so while the camera is tuned to pick up people up to 20-25 feet, it may pick up truck going by much farther.
My suggestion is to make the camera focus the area you want to focus, and make sure it pick up people walking in those areas. Depends on your camera angle and situation, it may pick up cars father away, but not people outside of the range.
It is a bit hard to visualize what you described but does this explanation match what you saw from the video ?