The typical reasons for underrepresented seat maps, tour groups or originating from a region where picking advance seating is not common, probably don't apply here. Plus the close in and recent flights don't look terribly different. And in this case, the UA display of seatmaps is giving the opposite impression, higher load factors.

However. the key point of the post was one can better see the impacts of seat blocking on EF than on the united site or apps. Looking at the united site only when with this COVID seat blocking can lead to a significant misunderstanding of the actual situation.

As for the high fare class / process, see the discussion in Ridiculously uncompetitive airfares?