If you are talking about the old policy, then yes, none of the airlines truly know what the "fare difference" was at the time of booking, or at the time you were downgraded, as they don't actually track or archive historical fare class availability on flights, they only keep historical fare filings. At best, they would only know the cheapest airfare that was available for purchase at that time based on the historical fare filings and factors like advance purchase requirements, roundtrip booking w/min stay requirements, etc. But they have no way of knowing if there was fare class inventory availability for that fare at the time of purchase or downgrade. Anyone who tells you they compensate based on fare difference is not being completely honest or giving you the full story unless they also explain how they calculate the fare difference. The new policy at least seems predicatable as it is based on 40% of paid fare and not some opaque calculation.