Has anyone had any luck getting it repriced?
My explanation of what goes wrong is this: the reservation system and the booking system are separate. This is why, when you go to purchase a flight, you sometimes get an error message. It's only at that point that it checks up-to-the-moment inventory in the booking system. The quote that the reservation system gives you may be minutes or even hours old, and in the meantime a cheaper fare class could have sold out.
But this is only my interpretation of what's happening. I'd like to know if DL will actually honor the lower fare.