The keyword here is Yield. Manufacturer produces X number of ICs, tests them, and finds out that Y are good. Yield is
That's one of the biggest issues with new processes, or when making more complex ICs (adding cores for instance). If the yield is not good, that means your manufacturing cost per functional unit is higher (because you're throwing away a large number of bad ICs) and your production rate (of good ICs) is lower. Altogether not a good thing.
Yields can be as high as > 90%, or as low as 30%. That makes a big difference!
An alternative to throwing away the "bad" ICs is binning:
- This IC has been tested to be 100% functional at the highest speed? It goes in the top-tier bin (most expensive).
- That one only has 3 out of 4 cores which are functional? It goes in a different, cheaper bin, sold has having only 3 cores (the 4th, non-fully-functional, core, is disabled prior to sale).
- This other one only works at a lower frequency? Same thing.
If you decide that your target is to have only 3 out of 4 cores fully functional, then suddenly your yield goes up. This is something Apple for instance have done for some of their processors which were sold as having 3 GPU cores, but actually had 4 on the die, but only 3 were active. That's a sign of a yield issue.
Note that the same applies to many other manufacturing processes, including LCD screens for instance. And of course it's nothing new... Even your fruit and vegetables are subject to the exact same processes!