I don't know the particulars of that truck, but I do know that when the ratings are so diverse it is usually due to frame and / or chassis components. I remember when comparing the '02 and '05 dodge Dakota ratings, they were about 1200 #'s different. Turns out it was the fact that the frame in the '05 was "boxed" vs. open, but the brakes and springs were essentially the same. The Drive-train remained the same also. I made the same comparison with the Dakota and Ram with equal drive-trains. The 'difference was about 4000 lbs. With that comparison, besides the frame, the brakes and suspension components were also larger. I will state though that knowing how the Dakota towed [pulling power considered] there is no-way that I would have towed a heavier load just because the rating was higher. While the Dakota did well, it did pretty much all it could, and adding 4k to that would not have been good. Again, I could be way off when it comes to the Ford, but that is what I have found with the Dodge.
I will note also that tow ratings do not usually consider wind resistance (both head and drag). The ratings are usually done with a low-profile trailer, and the "highway" speed is usually 45 mph. Calculations that I have done with my rig came to approximately 2800 [additional] lbs of weight due to wind at 60 mph. Mileage ratings are done at that speed also. I know not true to the OP, but wanted to mention it.
