Max tow might be the difference there. I don't know what all max tow adds versus my regular tow package. Heavier duty hitch receiver? But you don't have the max-tow mirrors? Doesn't max tow come with the towing mirrors?
I had to add the trailer brake controller to my truck but it's got everything else.

 

Towing mirrors are not included in the Max tow pkg and have to be ordered separately. The Max tow includes a heavier bumper/receiver assembly so could account for some of the weight difference over the 53A pkg. The 53C also comes with an upgraded rear axle and 36 gallon fuel tank. The larger fuel tank alone would account for 100 lbs over the standard tank.

 

Like old_programmer mentioned the tow mirrors are a Option does your truck have the 36 gallon fuel tank if not that's probably why your Payload package is higher like old_programmer mentioned?

 

nope, no 36g tank.

 

That must be the difference then!!!!