According to answers below this question, every Pi Zero is already "maxed out", but from my understanding this doesn't make sense. Silicon chips are manufactured with some variability in quality, and only those who can reliably run at 1GHz are made into a Pi Zero. This means some of the chips should be able to run at 1.1GHz or an even higher frequency.
How can they max out each individual processor?
You are correct, they are overclocked in the sense that the processors come off the same production line as the ones in other single core models and "binned" according to how they test.
However, this is not at all a phenomenon unique to the Pi or Broadcom; Intel and presumably most or all other microprocessor manufacturers do this as well.
But conventionally we would not call an i7 an overclocked i3, etc. The official clock speed of the chip is what it is sold as. "Overclocking" refers to exceeding that distributor's spec.
I've read before that because of this, people who are into overclocking for its own sake thus prefer the lower binned (ie. slower labelled) chips, because they have better overclocking potential. On the surface, this seems like faulty logic, since they were presumably binned the way they for a reason (it's also pointless if your real goal is a faster chip and not just seeing how far one can be overclocked). On the other hand, there's no reason to believe that the higher binned chip can be overclocked more than a lower binned one, but there is a little bit of a reason to believe the inverse because we know that some of those chips are sold with a higher clock speed.
So...
This means some of the chips should be able to run at 1.1GHz or an even higher frequency.
No, it does not mean that at all. It means you can try to overclock any chip you like and it may or may not work. If you aren't careful and it then breaks, you can't blame the manufacturer.
