Downgrade compensation under Regulation EC 261/2004 is calculated as a percentage of the original ticket price. In addition to that compensation, the PAX is also entitled to an invol. refund of the fare difference.
Wait, wait...
If the passenger is bumped, and not allowed to fly at all, then the airline owes:
full original ticket price
plus a fixed sum for compensation.
Right?
Now, if the passenger is reaccommodated in a lower class of service, the airline owes:
the difference between the full original ticket price and the fare for the class of reaccommodation
plus a fixed percentage of the original ticket price.
Again right?
Then suppose that you are bumped from an intercontinental F class because your airline does not have enough seats in F, or has to substitute a plane with no F class at all.
If the airline does not reaccommodate you at all, they owe full refund + a fixed and therefore capped number of euros. If they get a reaccommodation in coach, what they owe would be full refund minus the coach fare plus another 75 % of the original first class fare.
75 % of an intercontinental first class fare may well be more that the fixed compensation which is the same for all bumped passengers. It can also be more than the fixed compensation plus the fare for the intercontinental coach reaccommodation. Therefore, the airline would lose money by reaccommodating first class passengers rather than bumping them altogether...