My guess is that it shows the flights that DL wants you to take. For example, it might start with a bunch of nonstops, but the first ones listed will be at very bad times like 5:30 am or the flights that seem to be most empty, even at the same fare as other flights. Some prime time nonstop flights might not even appear on the first page, again despite sometimes having the same prices as flights on the top of the first page.