My flights today (12/8), FNT-DTW-LGA, cleared 6 days before. The FNT-DTW flight had empty FC seats while DTW-LGA was full and when I cleared EUA, only window seats were available hence other higher ranking PE got the aisle seats.
My flights on Saturday (12/10), LGA-MSP, cleared 6 days before but the MSP-LAX flight still has not cleared. The FC booking shows 1 empty seat today (whereas there were 2 at 6 day mark) while the availability site shows P2F1 hence the two may be saying the same thing except there are two seats in FC they are holding for battlefield upgrade.
Of my flights on Wednesday (12/14), MSP-FNT cleared today with plenty of FC seats available while the LAX-MSP hasn't cleared. The FC booking shows 7 FC seats while the availability site shows P7F4, so again they are saying the same thing.
I guess NW has experienced more people willing to upgrade by using miles or paying on these longer and meal serving flights. And they are using this to not upgrade to the full extent.

HJ