The problem has to do with wood
Kit manufacturers can't guarantee a special grain or wood density will be used to build / pack the model.
FYI, My H9 US 40 weighted 7 lbs ready to fly .... I scratch-built my own US 40 replica using Contest Grade balsa and the plane came in at 4.1 lbs ready to fly. All the dimensions are exactly the same !!
...The funny thing is I spent less money building my own plane than what the ARF model costs
H9 US 40 Wing Volume Loading: 10.12
My US 40 Wing Volume Loading: 5.93