I should probably wait for the pros to respond, but my guess is that NW is more expensive because it is trying not to cannibilize its own flight somewhere else.

For example, NW is the only non-stop between SFO-DTW. Thus, they charge a really high rate for a non-stop flight and don't make it any cheaper for a one-stop flight for fear that if they do, people will not pay their exorbitantly high price for the non-stop flight. However, DL code-shares a one-stop flight with NW and there are enough seats on that plane to keep the prices low, so DL (who doesn't fear the cannibilization risk) offers those seats cheaply.

my 2 cents