it's not stored digitally in a chip, it's stored in analog form in a resistor. the phone measures the resistance on oot and translates the resistance into a capacity value. Because it's analog, it will vary a bit each time it gets measured.

this effect is seen with first batch mugen 2400mAh. They have a thermistor instead of a resistor. A thermistor has resistance like a resistor, but the resistance varies with temperature. Thus, the reported design capacity with the mugen 2400 varies wildly depending on the battery temperature at boot. (omething like 17-20C gets close to 1300mAh.. Booting a body temperature mugen shows 500ish mAh design capacity...