My understanding is when these are made some cores are faulty and can’t make a full fat chip so they end up disabling the damaged parts and making a cut down variant. There is also the marketing side of things. Trust me, they have all the data to make decisions to maximise profits, you don’t need to worry about that.
There will be some perfect dies but not a lot, on the other side a few chips will have a defect somewhere important and be scrap but the vast majority will have like 80% of the cores available