What raises the GM 2500HD tow rating to 13k besides 4.10's ?

I have owned three GM 2500HD's, two with 3.73 and one with the 4.10. They were nearly identical in real world towing capacity. Since you will be towing in 4th or 5th gear with any trailer over 6,000 pounds, the final drive ratio is not a big deal. GM must be using an archaic formula from the days when we towed with three speed manual transmissions. That's when it really mattered. 6th gear and an empty truck, the 3.73's got slightly better mileage.