i thought about it, and remembered that the bilsteins i got did the same thing. i think the actual rebound action is determined by the springs. springs are what maintain a car's ride height. shocks and struts just dampen the the motion, so ur car doesn't bounce up and down endlessly like a rubber ball. the rebound control on a shock is the resistance to the "pulling" of the piston from the shock. i think if u had both the old and the new shock next to each other, and u pulled and pushed the pistons on the shocks, u just may feel a difference. the new one may be a little stiffer.