I worked for United Parcel Service for seven years from August 1992 to September 1999 (met my wife there). I never worked for FedEx, but there are certain standards that most shipping companies have to follow simply because of the way things are shipped.
This is pure speculation, but I have no doubt I'm right. Whether your Mac went by air or ground I'm willing to bet that whomever loaded it felt the weight and went with it. When they train you to load both air containers (L9, A2, M1) or a trailer they tell you "heavy stuff on the bottom". So, you start with the heavy stuff as the basis of your 'wall' and you build the wall on that foundation. Once you've built up you start on the next wall forward of the previous wall.
Whomever loaded this was probably unaware that there was a computer inside. It was heavy, it was large so they put it down first and stacked everything else on top of it. If it had been packed correctly to begin with this would not have been a problem, but with packages stacked on top of it, your Quad bore the weight of all those packages on top of it. An M1 container is larger and taller than an A2, which is larger and taller than an L9. A trailer is of course larger and taller than all of them. So, the weight may have been more or less depending on what the Mac was loaded into.
Additionally, packages rarely come down the belts at a leisurely pace. Usually it's a ********* and you're loading like a maniac trying to keep up. Sometimes that means packages that should have been put on the floor get dumped on top. Sometimes it means throwing packages in order to keep up. You sort it out later when the crush clears. Also, while UPS always stresses "hand to surface" and I'm sure FedEx does too, when you're unloading that rarely is the case. You'll see unloaders throwing packages on to the slides because it's just easier.
I know this is not a resolution to your problem but maybe understanding the process might help somewhat.