I have noticed this as well as different class availability. NW will show no K fare for a flight, but DL will show T class available for it. DL will show one ways at approx half the RT fare, but NW shows a OW fare higher than RT. DL prices a bit higher for the exact same ticket in the comparable fare bucket. And finally, NW will hide NW flights that are cheaper and show you flights for DL instead that are higher priced. Only knew of this by using travelocity to search and then had to spend lots of time to get NW site to even show the flight with a schedule only search. I often cancel for refund w/in 24 hours that can't do on third party site so continue to struggle with the NW and DL sites.