Hi, as a mechanic whith motor racing experience I will try to help you understand this a little better. To a certain degree you are correct about the lower springs being stiffer, as when they are manufactured they have to take into account the amount of travel available. So they have to be stiffer in order not to bottom out etc, however many other things need to be looked at i.e damper ratings for bump and rebound and also different anti-roll bars etc, these all affect the car in different ways. So to put it simply the shorter the spring the stiffer it must be as the rest of your suspension remains stock.
Hope this helps