I think they do this on purpose sometimes to keep it exciting. It costs the same to make both chips and Amd can use little "mistakes" like this to persuade people to purchase the card who otherwise would have bought the competitions. Youd have to be insane to spend 200-250 more for a 780ti when you know the 290 was unlockable and offered the same/near the same performance. At least that might be the logic.