I'm hoping the experts, including the EF representative, will chime in here, but what I understand is that EF shows bucket inventory while delta.dumb shows tickets available for purchase. For example, there could be sLUT seats on a flight (EF) but no sLUT fares (DL) because advance purchase sLUT requirements cannot be met. However, in this case DL would allow sLUT SDCs because the inventory is available.

Your example, however, is indicating the reverse of what I would expect, so maybe EF isn't totally up to date on availability.

Neither source shows actual availability of seats, although internet DL sources can apparently see load factors and compare that to the numbers of coach and FC/BE seats on various aircraft.

Your last paragraph suggests a married segment issue, which I hadn't thought EF reflected. AFAIK EF just shows inventory segment-by-segment, for instance listing two connecting segments on different lines so that you can immediately see which flight is the problem. I'm not even sure how you can make EF display inventory for an entire connection, as the first line of your OP suggests, versus segment by segment.

At first I thought I understood but now I'm puzzled too....

Have you tried to contact EF and ask them to look at this example?