The wastegate is held shut by the force from the spring in the actuator. In order to open the wastegate, the force from the spring must be counteracted and exceeded. This counteracting force is generated by the boost pressure acting on the diaphragm of the actuator.
If we assume for now that the spring is undergoing linear compression, then a certain amount of applied force will move it a certain distance. Double the force and you double the distance it moves.
So how does this apply to setting base boost pressures... shortening a wastegate actuator has the net effect of further compressing the spring
in the actuator. Because it has been further compressed, it will be exerting more force so a greater pressure will have to be applied to the actuator diaphragm in order to move it.
The other alternative is to use a stiffer spring- and this is the case with "upgraded" actuators. Rather than use a weaker spring that has to have more compression applied to exert the same force, a stronger one is used. This also has the advantage of being better able to hold the wastegate closed against exhaust pressures- and this is the real reason for using upgraded actuators.
The exhaust gasses are under pressure, and will therefore be applying pressure to the wastegate, which will tend to open it. What holds it closed is the restoring force of the spring in the actuator. Thus a stiffer spring will hold a higher pressure than a weaker one.
So, to answer your question more simply, an upgraded actuator generally has a stiffer spring that will resist the tendency of higher exhaust pressures under boost forcing the wastegate open.
Also, if you wind up a lower rated actuator the stroke of the actuator is reduced. If the APC needs to reduce boost quickly, if the actuator has been shortened too much it may be impossible to dump the boost.