I know that a police cruiser can "out-run" it's siren. So, it makes sense that you could "out-run" this gadget. If you're traveling fast, by the time the deer hears it, all you are doing is startling the deer and causing him to stop in the middle of the road instead of walking across it. At slower speeds possibly, the deer hears this gadget before he hears the car, alerts and has time to evade.

Just a theory - somehow, it makes some sense.