Well, we can't precisely be saying.
But we do know that the frequency of scents and other stimuli (rubberized bands and other noisemakers) is generally used to direct deer away from desired areas.
One other idea: it's been speculated that deer may not respond well to many of the common repellants because they don't have a strong sense of smell for skunk or fox urine. Deer might be able to detect such smells as threats, but not as deterrents if they don't carry much meaning for them. However, there's no hard data so this is just speculation until the next study shows up!