Why is my Android phone off by 3 seconds only a few minutes after I set it to the Atomic clock time?
Time on devices is an approximation based on the molecular activity of the components it is constructed of, especially mineral vibration. Some components and the software that estimates from it is better than others. Better phones with better components, or, better phones with better software are necessary for best time estimation. If the estimation is off for you, leave the setting for synchronizing time from the network. The responsibility of your device to perform flawlessly is reduced or eliminated.

See this article online which details how clocks on devices work.