It seems the issue here is that delta.com won't show AF coded flights in the fare search, it only seems to provide Delta coded versions of AF flights. The cheaper flight options combine AF coded flights with the DL coded flights. It may be possible to get the AF coded flights by calling Delta. Conversely, I've seen KL coded flights listed on delta.com. So if you see a cheaper fare elsewhere involving KL coded flights, you can often get them on delta.com by doing a search by schedule and selecting the KL coded versions of the flight. I saw this recently when booking an IAH-CPT fare through AMS. Orbitz would should a cheaper price with the KL coded flights, however, delta.com was not showing the KL coded flights in a search by price. When I did a search by schedule on delta.com, I was able to pick the KL coded flights and get the same price as Orbitz.