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If business class is sold out, then it's sold out, not sure what explanation you were hoping for? What you saw had no correlation with each other. AC033 on the 17th right now is 100% sold out in J. The last few rows in the Y cabin (41 - 44 except 41ABCHJK on the 77L) are always reserved for day of departure so they were never assigned and then reappear in your case; it has always been available.