The devil is in the details.
If you make a comparison between the GP100 and GP104 there are more differences and construction than the differences between GM200/GM204 and GP104.
There are differences, you just have to pay attention to where they are which make key differences in architectures.
AMD has had architecture cycle differences just as much as nVidia has.
The HD7900 series was a new architecture that went into R9 280 series.
The R9 290 series was a new architecture that went into R9 390 series.
The R9 285 series was a new architecture that went into R9 380 series.
The R9 Fury (X) and R9 Nano were new architectures and they will end where they started.
Just as they are introduced this year as Polaris 10 and 11 as well as Vega 10 and 11.
That's 4 architectures, based on the same line but actual separate architectures.
Just like Fermi to Kepler to Maxwell to Pascal, the difference being that the jumps between all those architectures were larger than GM200/GM204 to GP104.
It's very hard creating a new architecture when you've been pushing that to the limits for 5 years so there's 0 point in creating new architectures.