It is almost certain that there will be some people who won't show up for the flight and with J class wide open, it is a smart thing for them to oversell the flight... which is another reason why they closed off "C" class on this flight.

To Afterburner:

What do you mean in terms of "oversold" versus "sold out", and which terms applies to this particular flight?

Oversold=taken more reservations in a particular class than they are seats. However until the oversell capacity limit is reached, they will still accept more reservations. Once a flight is sold-out, they are not even accepting reservations at all.

Typically, flights are oversold by 10-20%.

In the case of this flight, it makes sense for the airline to take far more bookings than seats, as the overflow can be placed in business class.

Just out of curiosity I tried booking 4 seats on this flight. It accepts this booking in either J or Y class. So even though the flight is "oversold" (more bookings accepted than there are seats) it is clearly not "sold out."

This was explained perfectly by megamiles. The reservation system makes certain assumptions about how many confirmed passengers will not show and allows additional bookings accordingly. The system even uses different factors for different flights. Some flights tend to have more no-shows than others.

If you are also interested, try the web site www.trip.com . There you are able to see the flight in the air and exactly where in the world it is, the speed and altitude. Kinda neet.