i think it's the stiffnes of the chasis, mostly happens with shorter cars and if the shocks are short.

say you have a standard escort mk4 with normal shocks if you jack it up it goes up a long way before the wheel gets of the floor
but if you put some uprated shocks on there there will be less travel than the standard one's so the wheel will come of the floor at half the hight it would of with the standard one's thats the way i see it hope this helps