They're supposed to be? Really? Where does it say that? What source material are you drawing on?
In any case, Pallegina strongly approves of the Watcher if they're Diplomatic, but is personally hard-handed, hard-headed, and is honestly pretty violent. She's the essence of "walk softly, and carry a big stick." That's Diplomatic/Aggressive to a tee. I guess you could go Diplomatic/Passionate, since Passionate choices largely work out to "judgmental ass" in practice, but that seems kinda meta.
On a meta level, I can't be the only one who's noticed that the ten dispositions are easily and neatly divided into five opposing pairs.
But, beyond that, the mere definitions of aggressive and diplomatic put them into conflict, even how they're handled in the game itself. Diplomatic is all about negotiation, compromise, and avoiding tactlessness and hostility. Aggressive is forcing conflict to get what you want and refusing compromise. I'd never be able to reconcile those two as being at all compatible.
Personal opinion, perhaps, but again this strikes me as suggesting a paladin order that is benevolent and cruel. Just doesn't compute.