But maximum battery capacity should not fluctuate even if the battery capacity has gone below 80%.
Anyway, I am seeking for battery diagnostics service. Hope Apple could find out the true reason.
After a few hours of charging and discharging, maximum battery capacity, or we can say remained capacity, goes from 41% to 69%, then 76%. This is abnormal. Plus, this is not the battery percentage number we all can see beside battery meter.
That's exactly what happens. As the battery degrades, it doesn't charge as much as it use to. So that's normal behavior. When the battery is 20% consumed, it will only ever charge up to 80%. iOS will still indicate that it's charging to 100% but each percentage point holds less power.