If they’re that accurate then why does it fluctuate so much as the SoC increases? Shouldn’t it be one number top-down?Not quite. Apple doesn't report battery levels or battery health in detail in its GUI, those are just general (cheerful) figures. For instance, I have a 13 mini, which after 166 load cycles is still well above even its design capacity, see attached screenshot from coconutBattery Plus. Mainly because I have charged it between 20 and 80 percent only. Such a regime greatly improves battery performance overall. But the fact that the battery's full charge capacity is above its design capacity is just down to luck, not one battery is chemically identical to another. Now, please don't second guess coconutBattery's stats, they are legit (the app uses iOS's own internal measurements).
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They’re all estimates and without proper hardware you can’t get an accurate number. It’s all calculated using algorithms based on a few variables.