Delta Airlines bought out Northwest Airlines quite some time ago. There are no aircraft belonging to Northwest any more. They all belong to Delta now.
So why do I keep seeing Northwest Airlines as the owner of so many aircraft on my RadarBox? I understand outdated photos. The funny thing is, I often see aircraft listed as being owned by Northwest appearing in Delta livery in the photo display.
Same goes for Northwest Airlink, although that can be harder to track because of all of the commuter lines and subsidiaries.