It is similar to any stores "flash sale". If you happen to be in the store, you win!
Or Amazon having "daily deals", Kmart having "blue light" specials.
Other stations have "doorbusters". They all do it, and what it really means there is not a lot of quantity, OR they need to get rid of some inventory that is hanging out in warehouses and they are not re-ordering. (they = general sales online or brick and mortar, not just QVC).
Just another sales tactic. Some get lucky, some miss out. That's the name of the shopping game, imo.