The EU can come up with whatever contrived explanation it wants but the fact is that by the films themselves, nothing about either Death Star construction makes any sense. At the end of ROTS its about 25% built--you have the main inner superstructure, which is the most major section of the construction, plus a good start on the outer and inner hulls. So how long has this thing been under construction for? Since the ten year period when Palpatine came into office? We don't know. Then its complete twenty years later but it gets blown up and then in a couple years another even bigger one is nearly complete. Were they both constructed at the same time? How the hell did they build the other one so fast? Nothing is ever even mentioned about this huge development. You have clutch at straws and invent your own explanation to fill these holes--"oh they learned a lot from the first construction," "oh there was labor difficulties and rebel insurgents" etc. You might as well not even have plot continuity if you are just going to be doing this.