thing is the 980 is a true NVidia card the 970 is not
If you've already worked your way though our GTX 980 review, then you'll know that unlike that card, Nvidia is not producing a reference model of the GTX 970, the second enthusiast-grade Maxwell part that replaces the GTX 770 in the product stack. The images on this page do not reflect a ready to buy product. Instead, Nvidia's AICs will be able to purchase the GPU and memory chips, but after that it's up to them, and this will undoubtedly lead to a high degree of variability when it comes to PCB and cooler design as well as factory overclocks. [bit-tech.net]
Furthermore, as we mentioned in our GTX 980 review, GTX 970 has been a pure virtual (no reference card) launch, which means all of NVIDIA’s partners are launching their custom cards right out of the gate. A lot of these have been recycled or otherwise only slightly modified GTX 700/600 series designs, owing to the fact that GM204’s memory bus has been held at 256-bits and its power requirements are so low.
[Anandtech .com]