The beam pattern of the head is more horizontal than vertical, when mounted properly. You don't want to mount vertically because the head will be throwing a lot of IR up toward the sky and down into the road (which isn't the main problem) and not so much IR toward the sides (which IS the main problem). Jamming requires that the IR from the jammer reach the gun with sufficient intensity to confuse it. If you are putting the main beam pattern in an up/down fashion, you won't properly jam off-axis shots. Also, the receiver might be designed to catch shots to the sides (when mounted properly) more so than up and down.
See the beam pattern here:
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