Not that I'm trying to defend DL, but I just priced out a trip that I'm planning on booking within the next few days ATL-MEL and when I select the SAME flight numbers, I get exactly the same price through Kayak and through DL.com. After reading this thread, I figured that it was worth at least checking Kayak first to see if I could save some money and then go in and modify the reservation the same way I usually would (as mentioned in this thread). This leads me to wonder if the big price discrepancy might be tied to not matching the same stops, times, etc.