Im by no means an expert on Expertflyer. But availability on Expertflyer's seatmap just shows these people havent been allocated a seat yet?
I see this often on ET flights. Seatmaps shows most seats available, even on the date of flying, but when flying (or checking the seat map while at the gate) it is clear this is not the case.Presumingly because many people dont select a seat until they physically reach the check-in counter.