How does flying at ECON computed speeds optimise fuel burn.?
The key is the word 'Econ', which has nothing to do with 'Fuel Burn'. The 'Econ' setting is a MIN COST method of operating, taking into account not just the cost of the fuel, but also the other operating costs of the airplane.
If your hourly costs (maintenance, crew) are high, then you'd burn fuel to get there fast with reduced hours. If your hourly costs are low, then it's worth flying slow, and thus conserving fuel.