The default battery meter makes a randomish guess at the state of charge when it starts (at boot), its state of charge tracking is limite by bounds set to it by the battery size indicator resistor on the battery itself. In addition to a crude integrating charge counter, it also seems to have sanity checks based on voltage, which makes it periodically re-evaluate its integrating state of charge tracking.

If the meter is only slightly wrong, it'll just stay unchanged for some time until it thinks it's correct again. If it's very far off, like after an interrupted charge or comig from cold into warm, it might actually compensate it upwards..