I happened to look at some other simple return journeys (US-EU) yesterday for December/January and where previously every flight that was offered would show "Discounted" above the price like with the SEA-LHR-MAN options there were days and flights were no discounts were being offered.

Clearly either the statement that the discount will be offered on all published retail prices is no longer true, and/or there is an IT issue that is preventing the discount from being offered on flights where it should be. Golfmad's example of the full discount where the T&Cs state that none would be offered would appear to be another example of a change in conditions, an IT issue, or both.