Payload rating is often based on the tires that the factory chose to put on the truck and also includes a 150# passenger calculation. If the truck has a crew cab that holds 4 passengers there will be 600 lbs. subtracted from the "payload" rating for the pickup. This is only done with trucks and is confusing and never spelled out in the brochures.
I don't believe this is correct. The "tow" rating accounts for a 150lbs driver, not to be confused with the "payload" rating. The payload rating is simply the empty truck weight (including fluids) subtracted from the gross vehicle weight rating. Payload reduces by the actual weights added to the vehicle, not 150lbs per person.