We don't know in the scope of the six movies that it took twenty years to make the "first" Death Star.

It's entirely possible that there were earlier versions that didn't work, maybe they fired up the superlaser and it tanked the whole thing. Back to the drawing board as they say. Perhaps they made one that was sabotaged ("there'll be no one to stop us this time......").

What we do know is that there is 20 years between trilogies and that one is seen under construction at the end of #3. That doesn't mean that the one that you see at the end of #3 is the same one in #4.