It wasn't. In fact it was slower in many games and only faster in a handful of them. After 3 months of driver enhancements, it became consistently faster. Just go back to forum posts and YouTube comparisons from the time of release to see this.
I think you are confusing the 1070 with 1080. The 980ti was on par or faster than the 1070. On release the 1080 was around 25-40% faster (game dependent):
https://www.guru3d.com/articles-pages/nvidia-geforce-gtx-1080-review,1.html