At 18:00 when Adam and Jo are measuring the weight of what was in the hidden compartment of the van using impact marks on the vehicle's bumpers. One measurement is not taken in to consideration: Breaking Force.
The breaking force of any vehicle, including bicycles, causes the front-end to dip downwards (Newton's Law).
As this was a collision and there were breaking skid marks, one would assume that one vehicle had to have been breaking hard; this would cause the vehicle to dip downwards. So, using the calculations as they did based on average weight of a male and the height of the two impact points to determine weight of 533 Lbs is a very inaccurate calculation.