I never noticed that but I doubled checked the manual to make sure and you're correct. However, I think Darryl is right, it's probably due to the smaller tires as the drivetrain components are the same on all models without the tow package. They may have a max load of say 2,100 lbs whereas the Goodyear SR/A's have a 2,600 lb load rating.
I'm thinking about getting the BFG AT TA/ko's in LT285-65/18 D's (same height as the stock Goodyear's) when they hit production in a couple of months for the additional load rating and added stability for towing. If you look on the door panel it will tell you the gross axle weight rating (GAWR) for the front and rear. I have the tow package and GAWR's on mine are 3,300 (front) and 3,800 (rear) for a total of 7,100 lbs. Yet, the gross vehicle weight rating (GVWR) for my truck is 6,400 lbs becuase of the load range of the "P" rated tires.
Hope that makes sense...