Routine calibration is key. Although, even then, it’s a constant estimate — just a much better guesstimate.Reported maximum mAh changes with the % of charge. It's reported higher if the phone is at 100%, then it will report a lower value. Values are not precise anyway, we are looking at different estimates of identical batteries, which won't predict how they perform. I obsessed too with my 13 Pro at the time, the battery was always weak but reported mAh were very high, on par with other units that lasted more than my 6 hours SOT
To maintain accuracy, a smart battery should periodically be calibrated by running the pack down in the device until “Low Battery” appears and then apply a recharge. The full discharge sets the discharge flag and the full charge establishes the charge flag. A linear line forms between these two anchor points that allow SoC estimation. In time, this line gets blurred again and the battery requires recalibration.

Battery Calibration - iFixit Support
All newly-installed smart batteries should be calibrated as soon as possible. This helps your system get an accurate reading on the battery’s state of charge. W
