And the simple explanation to that is...
The
Defiant wasn't originally designed to be a Federation warship. Rather, when the series needed a new hero ship, Jim Martin dug up some of his old drawings on a ragtag Bajoran freighter, which he had deliberately designed to feature detail reminiscent of the Wadi ship, including the big ring on top and the wedgelike bow.
http://www.starshipdatalink.net/art/images/dd-4.jpg
In the above pic, the freighter has already gained some extra greeblies in place of the dorsal ring, but will soon regain it in these reworkings:
http://www.starshipdatalink.net/art/images/dd-7.gif
http://www.starshipdatalink.net/art/images/dd-8.gif
The ring detail is also prominent in the Cardassian freighter/patrol ship design from "Profit and Loss" and "Tribunal", later to become the
Hideki class small warship. One is prompted to speculate that the ring is typical of Cardassian ship design, and that the Wadi vessel which subsequently is seen several times in Bajoran hands and once in the hands of the Xepolite allies of the Cardassians is in fact a Cardassian design as well. That would make sense, sort of: Bajorans might have gotten most of their post-occupation ships by taking over materiél left behind by the Cardassians. Their triangular ships introduced in "Past Prologue" are seen in Cardassian use, too...
In-universe, the only mystery is how the Gamma Quadrant Wadi could have obtained a Cardassian or Bajoran vessel. But I guess we could argue that the very fact they contacted Bajorans is related to Bajorans going to them on a Bajoran ship and inviting them aboard. That is, the Wadi could be passengers on that ship, rather than crew.
Timo Saloniemi