Maybe nVidia changed their mind and decided not to shrink the transistors? idk. It's still Maxwell, which can be seen by the enormous graphics processing power the 970 and 980 are capable of while still drawing less power than the 770, 780, and 780 Ti.
They wanted to get ahead of AMD so they released early since there has been delays on the 22nm. The jump in performance is pretty much the same as jump from the 6xx series to 7xx. When 20nm comes out, there will be even more transistors on the chip which means an even higher level of performance at even lower TDP. Also, AMD's Pirate Islands will be on 20nm as well next year.
But then wouldn't the 20 nm transistors be a new GPU architecture and/or series of GPUs/graphics cards?
Also, 22nm -> 20 nm doesn't necessarily mean more transistors, it just means that the transistors are smaller, which is why it uses less power. Just sayin' :)
Not necessarily, it could just be a shrink, and when they shrink there is more room for more transistors which they always take advantage of. Always gotta be on top am I right?