Yes I was thinking this too.Probably because City have no intention of selling a single seat exchange ticket as a standard match day ticket for that game, and are instead selling them as part of an extortionately priced hospitality package.
Of course, that might change if our title bid goes tits up over the next few days!
Been checking daily for weeks for an extra 1 but now it says Sold Out which is unusual.
I expect any returns to the exchange from now will not be available to buy as a normal ticket