If you're intentionally disabling functionality in order to sell at a lower cost, you're not actually saving any money because you still have to manufacture the thing. It also (I assume) opens up a risk to someone finding out how to "jailbreak" the extra cores and now you can buy an i7 for the price of an i3. Is the cost of having three different manufacturing processes so large that it's not worth switching? Is the extra revenue from having three different versions of the same physical chip enough to justify the jailbreak risk?
See here: http://isca09.cs.columbia.edu/pres/09.pdf
Also here: https://www.anandtech.com/show/2721
“When AMD produces a Phenom II die if part of the L3 is bad, it gets disabled and is sold as an 800 series chip. If one of the cores is bad, it gets disabled and is sold as a 700 series chip. If everything is in working order, then we've got a 900.”
Hmm, but what if 3 cores are defective? If that can happen(?), then it seems one extra functional core is disabled to get to an even core number.
Apple's M1 GPUs are the first where I've seen the choice between 7 and 8 cores (as opposed to 6/8 or 4/8).
Perhaps there is fault tolerance hidden elsewhere, e.g. the neural engine might have 17 physical cores and one is always disabled. Although this seems unlikely as it would probably waste more silicon than it would save.
It gets sold as a coaster
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.
After manufacture, they test the individual components of their chips. These chips are designed in such a way that once they identify parts of a chip that are defective, they can disconnect that part of the chip and the others still work. (I believe they physically cut stuff with lasers but my knowledge is out of date). This process can also includes "burning in" information on the chip itself, like setting bits in on-die ROMs, so that if your OS asks your CPU for its model number it can respond appropriately.
Interesting side note: The same thing happens when manufacturing even basic electronic components like resistors. All the resistors that are within 1% of the target resistance get sold as "±1%" resistors, which means it's pretty likely that if you buy the cheaper "±5%" resistors and test them, you'll find two clusters around -5% and +5% and very few at the target value.
EEVblog did some tests[1][2] some time ago on ±1% resistors, and found that while his samples were fairly Gaussian and within the spec, the ones from his second batch were consistently low. That is, none were above the rated value.
So yeah, don't assume a perfect Gaussian distribution when using resistors.
So the best chips which have the least errors in the manufacturing process are sold as top tier. The ones which have more mistakes in them get their defective parts disabled and then get sold as lower tier ones
Instead of assuming, it's easy enough to confirm that CPU binning is real.
If you just have one price, you cut out people who can't afford it and people who can afford to pay more get away with more of the surplus.
If you have several prices and create just enough difference in the product that it doesn't change the expense much, you can suck dry every class of user.
Bit of an MBA trick.
We are talking about chips here so not the same bottle of soap with different labels, I'd call that a trick.