I think that 20% number started out as an estimated average over all designs and all loading conditions.
Unfortunately, some people have adopted it as a "cast in stone" standard for all fivers, no matter what else may be true. If their particular rig comes in at 19% and tracks badly, it is because the trailer is light on the pin. If it comes in at 21% it is nose heavy and the manufacturer should be sued or something.
I pull a Titanium. That is the Canadian trailer with the extended nose (out over the truck cab) and by the very nature of the beast it is nose heavy. The designers worked with the axle placement to help balance it, but five feet of trailer cantilevered out in front of the pin is going to affect weight distribution.
Every design from every manufacturer is going to have its own "personality" when it comes to weight and loading.
All of that is a separate issue from the brochure weights. You must keep in mind that the brochures are prepared by the marketing and advertising departments.
