2 Handed weapons count as 2 slots because 1 handed weapons come with shields equippable.

E.g. Paladin gets sword and shield
BLM: with 1 handed can wear a shield as well.
WHM: with 1 handed can wear a shield

So when you have a 2 handed it counts as 2 slots to counter pld,blm,whm's posisbilities of having a weapon and a shield.