This tends to be the case later on in a product's production run, as the manufacturer has fine tuned the process and worked out most of the kinks, the pass-rate of finished items increases.
At this point, yes they may lock down perfectly good high end CPUs to a midrange model spec to meet a production quota.
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.