impedance confusion

now dont get me wrong I do know what I am doing....but I have been wondering something for awhile now and wondering if anyone knows the exact answer.....When you wire a 4 ohm DVC you get 2 or 8 ohm....   but why when you wire 2 dvc subs you get 1 or 4 in series and 2 or 8 in parallel,   why is it that the subs value would not be added like a single DVC       does this have to do with the fact  the DVC"S are connected inside each sub?   but the coils are not directly connected from sub to sub.....    Its kina puzzling if you really sit and think about it