That's pretty universal among truck camper manufacturers. They tend to advertise the camper for one class truck lower than it really should be loaded on.
If you follow the truck camper manufacturer's recommendations most of the time you will be at least 500lbs overloaded, up to as much as 1500lbs overloaded once the "real world" sets in.
Just like the truck manufacturers with their tow ratings, camper manufacturers are working off the best case scenario. Sure the bare empty dry camper might be within the truck's payload rating, if you just load it and park it, but you don't camp naked and hungry in the RV dealer lot!
