You're correct. Ford advertise the max tow rating based on a "standard" truck. Standard truck usually means the lightest truck configuration in the line up. Add a crew cab and you'll loose about 300 lbs or so off the tow rating. Add 4 wheel drive and lose another 100 lbs. Don't look at the advertised number, you have to look at the GCVWR (Gross Combined Vehicle Weight Rating) then subtract the actual weight of the truck, cargo (options can be considered as cargo too) and passengers. The number you have left will be how much you truck can tow.
My 93 F-150 has a GCVWR of 11,500 lbs.
-5,300 lbs. (the weight of the vehicle with all options and wet)
6,200 lbs tow capacity.
-200 lbs for driver
6,000 lbs tow capacity
Adding more cargo and passenger will further decrease my tow rating because I can't safely exceed the GCVWR or 11,500 lbs.