The actual capacity of a battery decreases as the battery ages through an irreversible fading process. This capacity loss occurs both with active use, correlated with charging cycles, and with inactivity through self-discharge. The device may even come with substandard batteries. However, the amount of loss is linear with time and accelerates with increasing temperature. Smartphones do not typically employ sophisticated techniques for estimating the usable capacity with the basic solution basing on simple charge cycle estimation and then using an offline calibrated lookup table to find the state of charge or battery level given the charge cycle.
SOC estimation in modern smartphones cannot capture the usable capacity of the battery, and thus have unreliable SOC estimation. This inaccuracy manifests as sudden jumps in the battery level, and can result in unexpected shutdown of the device.