That is surprising, with all ST holders being forced to take theirs - should only leave a few thousand for members.
Yup, I remember reading that when the away club takes its full allocation (3500) for a league game there are only 4000 tickets available for members including the ones sold to the supporters branches.
I suspect Villarreal have only asked for the South East Quadrant (1800), so that would mean there would be 5700 tickets available to members for this game. I suspect there aren't many unsold but you would think the club would be able to sell that amount of tickets to members.
It shows why the ACS is so important in terms of maximizing revenue. Without it there would be 10-15k empty seats for the Villarreal game costing the club 500k+ in lost revenue.