I'm sure the bottom spring is stiffer but its not what gives you the handling and flatness. It looks like spring rate 1 is all coils working on both springs . Spring rate 2 which will be higher is thanks to the top part of the black spring going coil bound making the black spring stiffer so the effective rate of the two springs is higher. Spring rate 3 is black spring fully coil bound meaning your are just on the orange spring giving an effective spring rate higher again. So its actually the orange spring giving you the "flatness" and black spring adding the comfort. Lets say 60N/mm for orange and give the black one and arbitrary 120N/mm on all coils and 200N/mm when top section is coil bound. Using (k1*k2)/(k1+k2) for springs in series you get roughly Spring rate 1 = 40N/mm. Spring rate 2 = 46N/mm Spring rate 3 = 60N/mm
Interesting system. How much of the black spring is coil bound at ride height?