Nvidia squeezed like a 50 percent improvement out of maxwell just from a new architecture on the same 28nm node
Now there's a switch from 28nm to 16nm ff+ (and another architecture change, though we don't know how much different it is from maxwell)
It's not going to be 25 percent, 50 percent is probably lowballing it.
I'd also imagine they'd like to stay far enough ahead of AMD to not have to release another card on short notice or cut the prices on these cards dramatically. The 970 was good to Nvidia and I'd imagine they'd like to replicate it's success.