Those towing capacity numbers are simply inaccurate. Per owners manual towing cap for my truck is 9800lbs.
And that's because the 4L80E transmission holds it back.
Chevrolet made the 1500HD that year also. It was rated to pull 7,900 with the 3:73 rear end. They have it with a higher number than the 2500HD.
Look at the numbers for the same truck, but an EXT cab instead of CC. It shows a 10,600lb towing cap.
That's a 2,600lb difference going from a CC to a EC. That make sense to you?
Trailer Life Towing Guides have been know for a misprint to two!
2006 Towing Guide looks accurate.