The 1070lbs doesn't make much sense especially when you say the manual lists the 1520lbs payload.
It makes all the sense it needs to. The brochure lists a basic weight for the body style. They will post the highest weight available which would be a base model stripped work truck. Start adding options and that number drops quickly. The door sticker doesn't lie, it's what it weighed when it left the factory with all options, except things added later by the dealer which would reduce the payload number further than what's listed.
1500 Rams have the lowest payloads of all 1/2 ton trucks. Yes even lower than a Tundra. Get a loaded Ecodiesel and it can be as low as 850lbs. Less than my Honda.