As a CO Platinum who at times has been a very frequent NW flyer, my observation is that NW has been running Platinums starting 6 days out, and that's usually when my upgrade clears. In the past, we've been told that NW and CO do not show favoritism toward their own elites, and that CO and NW Platinums are processed together, and no preferential treatment is shown by the airline actually operating the flight toward its own elites. However, it's possible this has changed, or it's possible that you just happened to be behind some NW Platinums in the fare class pecking order and that NW didn't have enough seats released for EUA 6 days out to upgrade you, but had released more seats the next day, when you were upgraded.

FWIW, NW upgraded me a full 9-10 days in advance of my ATL-DTW flight this past Monday.