Nvidia has simply reached the point that supply and demand for 670/680 have found equilibrium, production capacity has met and surpassed demand.

I would wager that TSMC has been able to consistently increase capacity/volume since the introduction of the 28nm node as new equipment is brought online and the process matures, that is how they make their money afterall and how they pay for the new process ramp. The more volume the sell on a particular process node the faster they recoup their investments in the technology.

As TSMC 28nm capacity increases more products will be shifted to the 28nm node, I'm sure Nvidia and TSMC have coordinated and know what can be supplied and what projected demand should be ahead of release.

We'll just have to wait and see.