The game simplifies the actual historical reality. The real-life kingdom of Jerusalem contained over 20 seignuries, several of which were combined, created, abolished, split, or whatever over the history of the kingdom. They didn't divide up neatly into de jure duchies.; but in the game, they have to.
As far as I can see, the territory south of Jerusalem was an independent county, the Lordship of Hebron, that reported directly to the king. It wasn't in any "duchy" in real life. However the Count of Ascalon & Jaffa - referred to in-game as the Duke of Ascalon - did control a large amount of territory and smaller vassals in the south of the kingdom, and so Hebron ("Beersheb") is allocated to them in de jure terms.
There were four main nobles in the kingdom of Jerusalem below the king - the Count of Ascalon & Jaffa, Count of Oultrejourdain, Count of Tyre, and Prince of Galilee - plus several smaller ones as I've mentioned. The kings held Jerusalem, Nablus, Acre and Tyre as their royal demesne.