I think it's related to how long it takes NW to "close out" a PNR after a flight completes. You ever notice how it takes until 1-2 days after flying, before that leg of your reservation disappears from the "View/Change Reservations" section of nwa.com?

Just a guess - but I think that probably the WorldPerks computer systems and the NW reservations systems are quite separate. Th WP system seems to have multiple update runs throughout the day - so within a few hours after a mileage credit gets transmitted or downloaded to it from whatever the source system is (NW's operational system, DL or some other Skyteam or WP partner, Hilton, whomever), the WP balance gets updated.

DL's system appears to close out the itinerary as flown and create a FF update record or transaction very soon after the flight. If DL sends data to NW quickly, I'm guessing that one of NW's WP intraday update runs posts it.

But NW's own operational flight system doesn't close out the itinerary as having been flown for a day or two after the flight. Based on a couple of recent three-segment legs, I'm guessing it's about 5pm Eastern time - since I got credit for the 9am and 1pm segments the day after the flight but didn't see the 5pm boarding segment until a day later.

Totally anectodal and inferred from posting times, but as a large-systems computer geek, I'd guess that WP updates about three times a day. And that DL creates FF records within a couple of hours after the flight, and has an intraday feed into NW WP.