Manufacturing.
At the time yields on memory parts were below 20%. Meaning only 20% of memory parts would actually work. Solution? Manufacture 5X as much? Or figure out how its failing and build in redundant components.
For dense video chips you have shader modules, and memory cache interfaces.
Switching redundant parts is done when the chip is still on the die. If it can be brought up to snuff, its packaged and sold.
Watch an actual explanation, binning discussed at 3:43.
https://www.youtube.com/watch?v=b74MYv8ldXc
The impact of this defect is that memory access is 7 times slower on the 500MB of RAM. Perhaps, just perhaps this is why the 970 initially scored so low on performance.