ok, swifty is right mostly. But the reasoning for it is this

Your stock shocks are valved and setup to be riding at a certain height (ie stock). When you shorten the stroke, by adding springs, coilovers or whatnot, you change the geometry of what the shock was designed to do, creating stress and added ware as it trys to keep up.

Now with aftermarket shorter shocks (koni, kyb, tokico, blah blah) they are made for a shorter stroke, therefore greater response and compatability. Bsaically, its designed for a lowered vehicle, with different valving for stiffer spring rates as well.

-Grant