The real reason IMHO, is to have a larger range of product prices so you can cater for specific audiences.
It seems people are confusing cost with price. Those two things are orthogonal.
At this point, yes they may lock down perfectly good high end CPUs to a midrange model spec to meet a production quota.
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.
Either they just accept the fluctuations in order to maximize output of high end chips, or they would have to cripple fully functional ones to maintain a predictable supply. Interesting business.
The primary purpose is market segmentation: extracting value from customers who would pay more while not giving up sales to more price sensitive clients who nevertheless pay more than the marginal cost of production.