As Freebird mentioned, many of those are simply way over loaded. Some get away with it figuring that they are only going a short distance and or fairly level roads or just don't care.
There is a lot more to towing than engine size and rear gear ratio. Brakes, engine and trany cooling, axle bearings, tire ratings, hub sizes, even the weather makes a difference.
When you look at that total combined max towing weights, many fail to add in the weight on the tongue, the weight of fuel, passengers including yourself and any cargo in the truck including all of the stuff that you store in the truck like jumpers, extra oil, tow chains etc. Its all weight and it all counts.