Why confined aquifers have low storage?
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.