That is true but it does not affect the phenomena of sell-outs and empty seats.There are plenty of Season Ticket holders (two next to me) who often fail to show for midweek matches. I am in Block 232.
Seasoncards relate to league games. How many midweek home league games are there? One maybe two a season. The phenomena that people have noticed is groups of seats that are empty when the game is sold out. If a long-distance seasoncard holder has decided not to go to midweek Cup games then their seat is on sale. 99% of people behave rationally when it comes to money so I don't believe there are people paying for seats and not turning up. That happens a lot at Man Utd Cup games. I know that from discussing this with a City steward who works at both clubs. That problem relates to their policy of automatically charging their season-ticket holders for midweek Cup games, but those tickets are all over and sprinkled throughout the ground. What you notice in City's South stand is little groups of empty seats that are clearly not seasoncard seats.
I think City have concessions for academy players, staff, sponsors, visiting youth teams etc and sometimes they don't show, or leave halfway through a game as happened right on the front of the stand behind the goals a few weeks ago. I expect this is going on right around the ground and probably at every ground in the country but we notice it more in that stand because we are sensitive to it, and city probably operate more concessions in that stand than elsewhere because they are the cheapest seats. Historically I think City have probably been quite relaxed about giving concessions as we are a community club and we had a big stadium but now the pressure on seats is much greater than it was historically was but City are locked into past behaviour.