The low yeilds that people speak of is because when Nvida makes the cores, they sort them... since they are only making 1 version... the ones that come out right become 680's the ones that come out FLAWLESS, they get shelved, destined to go into a 690 card with a well-matched partner... or sometimes get sold to card manufacturers to become an "overclocked 680".
The mistakes... well, if its only 1 bad part - those will become 670's with the faulty silicon disabled. 2 mistakes makes a 660, and so on. If it comes out too messed up, it gets meleted down and recycled.
So yes, its realistic that it will take them 6 month's to make enough mistakes to have a decent supply of the "lower" cards at launch. This is a common practice among chip makers, and saves them money on manufacturing then say printing 6 different chips - they just print 1 version and the "slight defects" get sold as lower models.