Airline seat availability is much more complicated than just the question of whether the number of reserved seats equals the capacity of an aircraft. Airlines use seat availability as a way to dynamically adjust prices according to demand. Simplifying somewhat, each published fare is assigned a letter of the alphabet called a booking code, typically also the first letter of the fare’s basis code. The airline chooses the booking code for a fare based primarily on the fare’s cabin (coach, business or first) and the fare’s price.
Asked whether there are any seats available on a plane, the response an airline gives is not “yes” or “no” but rather a per-booking-code vector of seat counts. For example, in the figure the first flight has 1 F booking code and 4 H booking codes available; the second has no F’s and 3 H’s. To fly on these two flights using the H14ESNR fare (with booking code H), H seats must be available for both flights. They are, and up to 3 people could buy H fares, but a (cheaper) fare with booking code Q could not be used because no Q seats are available for the first flight.
Airlines do not usually publish seat counts higher than 9, so even when a plane is empty it is common to see F9 Y9 B9...