ATL
For me, NW Plat, I've only been EUA about 30% of the time when traveling through ATL with about 50% upgrade success. I've also been as high as #34 on the upgrade as plat there.
While elite passenger load vary from flight to flight, it might be possible you are booked on elite heavy flights.....
As for SEA-DTW, I'm about 50-50 as plat on non-redeye flights. Success rate is higher for the redeye flight.
Jiburi