Difference in winding. I'd have to measure the distance between coils to to give you a more detailed summary of this.
2017 # "effective" Coils. The coils on the ends will not be an effective coil, it will be there for mounting/height. The effective coils are in blue: 4.25 coils are effective or active. The pink is the non-effective coils.
2017 RWD below with 4.13 effective coils.
View attachment 90503
This one is a bit harder to see as I don't have a pic of the QV springs separated. 3.70 effective coils. The other coils are dead coils that give height to the car.
The increase in spring diameter is doe to having more effective coils which raised the vehicle.
QV 2017 coil Dia. 13.60+/- with 3.70 effective coils. Spring pack 79
QV 2018 coil Dia. 13.80+/- with 4.66 effective coils. Spring pack 64mm ( I am wondering if this is a typo and should be 94mm)??
(hence the almost 1" height difference but with similar performance.) Spring pack most likely due to how it was wound.
2017 3.70 effective coils:
View attachment 90509
Q4 2017 (non-sunroof) 12.80+/- .05 with 4.25 effective coils. Spring pack 84mm
Q4 2018 (non-sunroof) 13.20 +/- .05 with 5.11 effective coils. Spring pack 98mm
Not only almost a mm thickness change but a .86 coil height difference, close to 1 full coil which is about 1". We can see that in the ride height of the 17' vs the 18'.