why is the Wii 7th generation

[LowlyAssassin]

I have been reading and I can't see why the Wii is a seventh generation console other then it's release date

I thought consoles were poor into generations based on bit processing power, and processing advancements, so 8-16 bit was the change from one green to the next
Or the graphical power signifying the transition from one green to the next


So why it's the graphical and computational in paired Nintendo consoles classed as Wii=7th and wiiu=8th ?