I'm confused. Is there more than one Duke of Wurttemberg at the same time? In any case, this certainly looks like an April/October marriage. Oh well, if they're happy . . . .
It's the way they style the members of the house. Duke Johann of Wurtemburg, vs. the Duke of Wurtemburg. Wurtemburg was a dukedom before the reshuffle after the Napoleonic wars, and then it was elevated to a kingdom. After 1918, they reverted to using duke.