Here's a picture of how a wafer is used to make chips. Those spots are defects. Each chip that has a spot makes it broken, and can't be used. The bigger the chip, the more likely a defect will occur on it. And the 1070/1080 are fairly big chips. Unlike the RX 480's which are much smaller. And since AMD's 14nm FinFET came from Samsung, which has a lot of experience with manufacturing. So AMD's yeilds are higher, even though they have very low clock speeds compared to Nvidia's 16nm.