25,000 miles is the DL price of an upgrade from a M or B fare; upgrading from a full Y costs fewer miles, but 400 Euros is likely to be less than the difference between a T and Y one-way SFO-AMS. The SkyTeam upgrades require full fares (Y) to use their upgrade with miles policy.

My reasoning is that either this could have been a DL buy up to M and use miles to upgrade process, except that the 400 Euro price seems improbable: such an even number in Euros and there should have been a $250 change fee in addition to the price difference, so that the total woiuld have been more than 400 Euros.

My best guess is that you got some sort of a special deal to sell seats in a pretty empty cabin. Normally these take the form of a flat rate (400 Euros) for the seat, but paying miles in addition is new to me.