Well eventually as yields improve you start handicapping perfectly valid chips to main market segmentation. I cannot say this for certain in CPUs but I know in

Well eventually as yields improve you start handicapping perfectly valid chips to main market segmentation. I cannot say this for certain in CPUs but I know in other electronics with PCBs that this is how it is done. Sometimes lower-end SKUs are made by opening a higher-end one and cutting a wire or a trace.