Fifth wheel Towing Help

While planning on an upgrade soon, weight ratings have me baffled. I have plugged in the numbers on some weight calculators and they come up short of what I can tow including the trailer I am towing now!!! But, it gets better, the weight ratings show I am fine towing what I am now and the scales show me way in the good also. So here is my dilemma. Tow vehicle is a gasoline 2010 GMC Sierra 4X4 extended cab regular or short bed manufacture GVWR of 9900, 3:73 ear ratio. I am towing a travel trailer at 6400 lbs now without any issues. So the plan to upgrade to a fifth wheel with a GVWR of 13900 and hitch weith at 1955. Manufacture shows a GCVWR of tow vehicle at 16000. Going through the checks and balances of different calculators and using all the info I have I come up short of towing the new fifth wheel. Now the question is how come everyone is pulling a large fifth wheel with the weight ratings that don't work or not within spec? I am not one to ignore the specs, but just about ready too unless I by a 2 ton truck! I am sure my truck will handle the weight etc, tow ratings jump with different gear ratio and / or diesel so the truck is the same, just mechanical difference so fuel mileage will suffer is the main issue only? Am I crazy or what? Honest input will be helpful. I will get with some folks at PD commercial enforcement to help as well. Your input is welcome and I thank you in advance.