Identical drop in head in unconfined and confined aquifers releases different amounts of water. The confined aquifer releases less water and thus has less specific storage. When both aquifers are fully saturated then what makes storage in confined aquifer lower as compared to unconfined?
In an unconfined aquifer the 'storage' is that fraction of the saturated void space that can be drained. In a confined aquifer, what you are effectively measuring is the compressibility of the aquifer - that is, the water released by depressurizing the aquifer per unit head. They are really two different things. In an unconfined aquifer, the water that is drained comes from a narrow cone of depression. In a confined aquifer, although the numerical storativity is very much less, the cone of depression extends very much further because it is a general pressure release which propagates far into the aquifer, in all directions, so the yield can still be very high.