I think this happens because the Nobility system is tied to the career mechanic. If you have a married couple, only one of them actually needs to be in the Noble career, since the spouse will automatically inherit the corresponding title. For example, if a female Sim is a Queen through the Noble career, her spouse will automatically become King, even if he’s not in the Noble career himself.
However, if both Sims are in the same Noble career track, the game seems to interpret them as rivals instead of spouses sharing status. My guess is that the Dynasty system might be a workaround for this — but I’m not completely sure about that.