I’ll try to hazard a guess here. Looking at the SCT route calculator website there could be an answer.
http://aplicaciones4.sct.gob.mx/sibuac_internet/ControllerUI?action=cmdSolRutas
A pick and a car are charged the same. But a two axle trailer is charged more. So maybe there is some question in the toll person mind about whether it is a truck or a pickup. Maybe some consider that duals qualify as a truck and not a pickup.
We are sometimes charged as a bus towing a car and sometimes the same as a truck with 4 axles. Now I look at the rate chart and see that the also rate buses as 2-3-4 axles so maybe they see us as a bus with 4 axle?
Reading what the sign coming into the toll might say or what this calculator might say does not matter in some cases when the toll taker defines you as something else. We would much rather be charged as a bus with 4 axles than a two axle bus towing a two axle car.
Below is a quick example of what the online toll calculator listed as tolls between Nogales and Guaymas (they may not show all tolls). Pickups and cars are charged the same – and all trucks and buses between 2-4 axles are charged the same.
When they say Auto One Axle or Two Axle trailer is that amount added to the charge for the pickup or truck (or car)?
.........................toll 1 toll 2
Trucks 2-3-4..............35 110
Pick up or car............25 65
Bus 2-3-4.................35 110
Two axle truck.............35 110
Auto one axle trailer......40 100
Auto two axle trailer......55 135
We roam all over Mexico and find the way the charge to be inconsistent as a bus towing a car. Perhaps there are also inconsistencies in how they charge depending if they consider you a pickup or a truck. What we in the states call a pickup may not be the same as how the define a pickup in Mexico. Perhaps when they see duals that upgrade you to being a truck – which raises the rate?