IIRC, GMT800's also changed the front suspension on 4x4's by having the bump stop in constant contact...so as to become another spring
So, when you measure with more head tilt, that 2nd spring comes into play
It also depends on that bump stop's spring rate. Since rubber/plastic...it might have a much more aggressive curve
Also, that POS reciever bends ever more as you load it up...so it will become another spring (reciever cross tubes are torsion bar springs on all receivers)
Meaning...it will unload and re-compress during whop-d-does, which will be somilar to proposing
Tire PSI is part of the suspension. More so with higher profile and less with lower profile tires. Meaning playing with tire PSIA will play