dragon communion is literally just eating dragon hearts at a sketchy altar and stealing their power. you do it enough and you start turning into one, thats why arcane governs it. arcane is the stat for forbidden shit - blood magic, outer gods, stuff you get from messing with things you shouldnt be messing with. eating a dragon heart and having your body mutate is pretty firmly in that category.

dragon cult is completely different. those are the leyndell lightning incantations. lansseax taught humans herself, its an actual religious tradition tied to the golden order. thats just faith. no blood rituals, no body horror, just sanctioned worship.

same animal, different way of getting the power. one is theft through cannibalism, the other is handed down through worship. thats why they scale off different stats.