1. This is quite normal behaviour of airline yield management systems. You're not watching tins of baked beans sitting on a supermarket shelf. The display is not of the number of seats left to sell, but of the number of reservations that the airline is prepared to take (bearing in mind that the cabin may already be well overbooked).

2. You can't see that availability has dropped by 11, because you can only see up to 9 seats. J9 C9 means only that the airline is prepared to take at least 9 reservations in one go, in either J class or C class. It does not mean that the airline will take 18 reservations. Moreover, if you book two seats, you could find that availability immediately goes (for example) to J5 C3. Remember: you are not watching cans of baked beans.