None of this makes any sense... The 670 is a cut down GK104, the full dies were going to be sold either way. The base clock for the 670 is 915MHz. I'm not sure where you got 1.1-1.2GHz, as even the 680's base clock is 1006MHz.
I contributed to the rumour threads on the GK104 in the UK. The initial leak pictures only showed a GTX670 SKU early on and it was a different shroud. The boosting mechanism was only really finalised late in development - people running the leaked samples were seeing clockspeeds under 900MHZ at max.
It appears,that Nvidia were tweaking clockspeeds as much as they can - that is why review samples were hitting as high as 1.2GHZ(and I had conversations with one reviewer about the boosting mechanism).
There was even a comment by Nvidia PR about the Tahiti launch. I have the feeling they expected to Tahiti to be faster,and the GK104 not to be able to compete,but Nvidia probably realised they could tweak the GK104 further.
AMD even admitted that they had been excessively conservative with clockspeeds. You can see with the HD7870 at launch the various models were running at 1GHZ to 1.1GHZ instead of just over 900MHZ with Tahiti.
Hence,why we all of a sudden had a 300MM2 part being the top card - this had not happened for years. The 8800GTX,8800 Ultra GTX280,GTX285,GTX480 and GTX580 were all massive parts.
Unless AMD is set to destroy Nvidia in perf/ mm2 (a reversal of Maxwell vs. Fiji, Hawaii and Tonga), then they will be best served to just release ASAP instead of holding back. The only adjustments AMD will have to make with launching after Nvidia is on pricing.
This is what I am thinking too - they must be trying to tweak clockspeeds,since they might be thinking it is more competitive than first thought.
OFC,it could be Global Foundries/Samsung might be having production issues,since Semi-Accurate reported Polaris was delayed and meant to launch earlier:
http://semiaccurate.com/2016/03/22/why-did-amd-slip-polaris-and-vega/
This means either Polaris 10 is being tweaked more since it is more competitive than AMD thought or the other end of the spectrum they are having production difficulties and needs more tweaking in that regard.
So it is one or the other,which are at both ends of the spectrum.
However,since P11 is a much smaller part and is targetting notebooks where AMD has been utterly destroyed in by Nvidia,I have a feeling its the former.