2015 GMC Yukon Denali XL. I only got 1-1/4” out of my front Belltech 2” drop spindles as well.
These drop spindles are stamped and/or molded spindles. They should be pretty identical.
Tony has posted numerous times that for some reason with the K2XX they have used at least (3) different spring rates. I think that has more to do with the inconsistencies of the amount of drop the spindles provide from truck to truck.
Springs would have no affect on the net drop from the spindles. With springs, there's plenty of room for variations- Inconsistencies in spring manufacturing, equipment on board that would vary the weight of the vehicle, age of the springs, etc.
Spindles are a "hard" part. They could be viewed the same as setting your vehicle's frame on jack stands. If you set the height from the floor to the pad of the jack stand at 12", then your frame will be 12" from the floor. Not some range of 10 1/2" - 11 1/2", etc. These spindles are made in such a way that you net a set amount of 1.25" (or 1.5"- maybe it depends on who's measuring) drop.
Is the mounting location for the hub only 1.25" higher instead of 2"?
Is the hub mounting location actually 2" higher but something else in the design of the spindle is negating 3/4" of that? Maybe the tapered hole that accepts the lower ball joint stud isn't drilled as deeply as on the stock spindle?