Greg, If you read the theory of operation for the motion sensor on the Yeti website, you will see how the motion sensor works. The sensor detects heat in movement from side to side. That is why there are vertical and horizontal boards. The way the motion sensor is mounted on the board is the only difference. Sometimes when I am testing my camera, I have noticed the sensor does not see my hand when I move it up and down in front of the sensor. There has to be some movement from side to side. Read the theory of operation, and I think you will see what I am talking about. The sensor misses very little, but if the movement is coming straight toward the camera or moving straight away, the sensor will not detect it.
The sensor beam is like a flash light beam. The farther from the sensor the larger the detection area. I like having my cameras out in the open away from trees and shadows because I think the pictures are better with more light.
Bob