A friend of mine is an engineer in the auto business. His buddy is an engineer on the Ford Superduty line. He said the ratings the mfg gives are what the average person would feel comfortable towing with the truck. So I do no know if you can call it a fudge factor but be reasonable when you tow.
Some of those who say you can only tow 80% must have forgotten where that rule of thumb came from. It was that 80% was your rig and the cargo in your tow vehicle would account for the other 20%. Now I know that I do not have 2400lbs of cargo in my truck.
And if you look at some of the specs for a F250 vs a F 350, the F250 actually has a higher cargo limit. Now, of course, you can buy a larger cargo package...