I think there are some exceptions. At one point anyway a horse trailer could be higher, and I think that is still the case. And RV trailers can be higher also. Most likely because more lawmakers have horses and RVs and they make the rules.....makes no sense otherwise.
Just to stir things up a little, California also limits a "pickup" by definition to a max GVWR of 11,400lbs. If you have a late model 3500 Dodge dually you are over that and even with a factory bed you are legally required to stop at the scales, even empty and not towing anything. Obviously the intent is not to have "pickup" in the scales, but it appears that some of our laws are lagging behind the reality of the newer trucks with higher capacities. That also means a guy hauling a >10K flatbed trailer with a "pickup" rated >11,400 not only needs a CDL but if a business must be in the BIT program and have his terminal inspected, do drug tests, etc. It's a crazy low entry point into what is meant for big trucks. California, land of the free......Just to be clear, I'm fine with the terminal inspections and making sure everything is safe, I just don't want to have to dedicate a bunch of time and resources to a program if I don't need to.
With our delivery trucks we have learned the California laws as they apply to us. So now we pull our big gooseneck with a SRW pickup, we sold the dually even though a dually would be marginally safer with the heavy pin load on a gooseneck. It still requires a CDL, but no scales or inspections or BIT program. And our 26,000GVWR (not 26,001) Freightliner rollback (without a trailer) with airbrakes can be driven by anyone, no CDL required, but must go through the scales. We can hook a trailer of less than 10K GVWR to the back and be ok, except if we are over 40' we would be back into the BIT program....and the truck is 31 foot already, so we never pull a trailer with it.
I bet every state has its nuances. We have found the best method to figure these things out is to just call a commercial CHP officer and have him come out and teach us. We have found them to be super nice and they like proactive businesses that ask them to be involved.