Marc...that is exactly what is meant by "apparent"...or observed stiffening. You are not actually making anything stiffer by cutting the spring. It does not re-temper the spring..or increase the cross section. What you are doing is taking away the progressiveness of the spring. You are in effect...shortening the amount of time the spring is able to travel before it reaches maximum compression. That is actually a change in enertia and leverage. This works in the short term, but leads to early failure...as you did nothing to increase the load bearing capacity of that spring. In fact by removing mass....you decrease the amount of total weight/compression that spring is able to handle.
Torsion or sway bars are the same. Yes..."twist" is a function of cross section...but is not the total equation. It is also a function of total length.
Length=leverage. Mass = loading capability. By shortening the outer arms...you decrease the amount of "lever" the suspension arm is able to use to twist the bar. The "apparent" result...or physical feel...is that the bar got stiffer. It in fact...did not. Just that with a shorter amount of arm to use for leverage, the suspension lost some ability to twist the bar. That is how adjustable outer links work.
The same can happen when you shorten the actual bar length. Less total surface area to be twisted. Total area is diameter x length. The "apparent" torque...or the felt torque "seems" stronger now, because the same amount of force is being apllied to less total surface area of spring steel.....and because of that fact....the bar reaches its maximum torque rate faster, It becomes quicker reacting. But in fact...you have reduced the load/twisting capability of that bar. Ray