Tony,
I cannot find the details of how it is worked out now, but have seen it somewhere in the past. There was a predetermined formula for shipping, and the weight appearing to be much higher than you would expect, is due to the volume. To put it another way, going from memory, this is how I remember it, a given volume has a standard weight to it, so if your packing crate or vehicle is x amount of volume, then that is how the shipping weight is arrived at, this is why it very often appears to be higher than actual. This way the shippers only need volume to arrive at a ballpark weight.