The initial allocation the club put onsale were sold out, but they do not put all the tickets onsale at the beginning of the season.Virtually every match was sold out in August. The vast majority of spare seats are no shows from season ticket holders who can’t be arsed or can’t attend for valid personal reasons.
They hold back the ticket partner allocation and a further percentage for players , staff and other hangers on.
These are the tickets that are then released back onto general sale, about 4 weeks before a match, when their partners realise they can't shift them.
Couple that with tickets exchangeses and you end up with 4000 tickets remaining unsold .
Edit,sorry M18 beat me to it