Can you spot the difference that can make a 40-80 kg different to an isometric dead start concentric only contraction?

Did you sport the difference? The bar was 10 cm lower in the picture on the right, which changed the joint angle of the start and therefore onset muscle length.

This subtle change in the standardisation of the procedures can have dramatic effect on what people lift i.e., their force expression. Why?

The force-length relationship holds the key, as in the amount of actin and myosin overlap determines the amount of force you can produce. This relationship is hugely significant in understanding everything from mechanotransduction and tissue remodelling, to standardising assessment and isometric programming. For a top up on this relationship and isometrics click here.