The spelling sometimes is written like it sounds. Plus in the early 1800, only the priest knew how to write. For example your last name could be a variation of:
Drouin, Derouin, De Rouin.
Just like my last name can be spelled in different ways:
Leblanc, LeBlanc, Le blanc or Le Blanc. And I have seen Leblank, Blanc even Leblanco (Italy).
Are you all from the same blood line? If you go back to your original ancestor from France he could be the brother of another Derouin who his descendants are in Canada but never reach the US like yours did.