David,
From my understanding, as the aspect ratio of a tire increases, i.e., sidewall measurement gets larger, the contact area gets longer and narrower, which gives a softer ride. As sidewall measurement decreases, the contact area shape changes to shorter and wider, giving better performance on cornering, but at the price of comfort. Even if the area of the contact patch remains the same, the shape of l x w has changed, which accounts for differing values in traction.
Just as with most suspensions, most street tires are a compromise between performance and comfort.