Hmm...I think it has something to do with referencing other specific cards or card types:
Band of Misfits=card costing less than this
Beggar=Copper/Silver/Attack card
Catacombs=card costing less than this
Conspirator=action card
Cultist=Cultist
Feodum=Silver
Fortress=itself?
Horse Traders=Attack card
Hunting Grounds=Duchy/Estate
Lighthouse=Attack card
Market Square=Gold
Moat=Attack card
Quarry=Action cards
Rats=Rats
Secret Chamber=Attack card
Squire=Attack card
Tunnel=Gold
University=Action card
But there's also something else to it because like Border Village, Trader, Urchin, etc. there's gotta be lots of other cards which reference cards or card types, plus I don't know why Vagrant would only be honorable mention.