The question is also about the application of taxes. If the 10% or 15% is off the fare only, but taxes apply to the full fare, the difference may be up to 5%. I recently tried my 15% off cert on a SNA-LHR flight in Novembert, and found a regular W fare of $666 and change. Adding in the 15% off cert made the new W fare about $600, only about 10% to 11%. I just assumed it had to do with taxes and airport fees, which I know are outrageous in the UK. Wrong?