Intresting online AC booking

It's amazing... Two days ago, I could not get a seat in the economy from Shanghai to Vancouver; AC forced or rather "enticed" me to get a Biz tix (13 odd seats). Today, those 13 odd Biz seats disappeared from the site but tonnes of economy seats available, almost the entire back of the 777! Go figure! After I booked my econo seat, I was able to use my voucher to request upgrade. Umm... Wonder what kind of scam it is running; I know there are lots of people traveling back and forth; but overnight, 13 Biz seats were sold? Not that I am aware there is a huge Chinese delegation going to Vancouver for a "conference".