Trigger outside of motion sensitivity

We have a stick up cam on the fairly busy street side of our house. Oddly we have found that the only 1 car outside of the motion zone triggers our camera. The owners of this 1 car live a block away and pass our house at least twice per day. 50-100 cars can pass thru, outside of the zone per day, and not trigger the camera, its just this 1. Can anyone tell us why? We dont want to change the sensitivity because otherwise it works perfectly.