It is the capacity published by Apple. Apparently some regulatory bodies [1] require that information.

[1] https://www.mysmartprice.com/gear/mobiles/mobiles-news/apple...

But it is an interesting question: What is the manufacturing tolerance? Naively I had assumed that it's much smaller than a watt-hour. But that's based on ~no knowledge :D

You can still have a specific "normal"/expected capacity, even if you're producing them +-20 mWh.

Surely the battery is over-provisioned to be able to deliver the stated spec for at least some warranty period fraction?

Well it could be that the capacity is 3200-3349, and they publish the number that is in the center of confidence interval. Rounding up would be received badly, and rounding down suggests it has less capacity than it really does (on average)

It’s a nominal capacity that’s printed on the battery, and encoded into its firmware. In practice, batteries tend to exceed this nominal capacity when they’re brand new which is why you see new batteries stay on 100% charge for a long time. As it ages, the true capacity will degrade and eventually falls below the nominal capacity.