How did you establish that there were 20 seats available? If you did it by counting the number of unassigned seats on the seat map, this is not a reliable method. Many people will book a flight and not ask for pre-assigned seating, so even though you see what appears to be 20 "open" seats, the cabin could be virtually sold out. You then check back later and the last few seats sell, so you can no longer view the seat map - if you were able to view the seat map for the sold out J cabin, it would probably still show many as open even though none are available.

A better way for you to check is to use a tool such as seatcounter.com or KVS - they show the number of seats available up to a maximum of 9.