Most clocks drift. If you've ever used a normal watch (not a smartwatch connected to a phone) or a battery powered clock, you may have noticed that you need to correct the time every month or so, as by that time, it's lost accuracy.

The problem is due to the quartz oscillators these devices use, which are the same ones we use in phones, computers etc., which have the same problem as a result.

You don't notice this because just about every network-connected device these days uses NTP or something similar to keep its clock constantly up to date, but the clock itself is still inherently inaccurate.

There are also other mechanisms to keep clocks in sync by the way. Some mains-connected devices keep time using the 50Hz/60Hz mains voltage. Various countries have radio broadcasts that devices can be used to keep time (DE, US and JP run them, I believe).