Why Did Two Theoretically Equivalent Methods Produce Two Very Different Values

In 1991 a team from the Universities of Newcastle and York conducted a study that set out to estimate the value of preventing non-fatal road traffic injuries. The research was commissioned by the Department of Transport (DoT) and the Transport Research Laboratory (TRL) to put the valuation of preventing such injuries on the same willingness-to-pay footing as the figure used to value the prevention of road fatalities.