OK - I should have googled myself instead of asking. I found this:
"21 Day Super Saver" Rate is a non-refundable advance booking rate that gives at least a 20% discount off the Best Flexible rate at time of booking. Reservations on the 21 Day Super Saver product are final and require full prepayment for the entire stay at time of booking with the exception of German hotels where cancelling your reservation or failing to arrive will result in forfeit of prepayment less estimated 10% of booking price. The 10% refund is repaid to your credit card. For all other hotels in Europe, Middle East and Africa Payment is non-refundable
It's the t&c's for another rate, but it is clear that it is for Germany only.
Still I would be interested to know the reason for this exception.