Back in 2015, Apple's Kevin Lynch explained that the timepiece is so accurate you can hold any of them side-by-side and you will see the second hands move in perfect unison.
Lynch explained how this was made possible.
"First of all, we’ve curated our own network time servers around the world," said Lynch. He says they've scattered 15 "Stratum One"-level Network Time Servers (NTP) (one level down from an atomic clock) around the globe. The servers are in buildings with GPS antennas on the roof that talk to satellites orbiting the earth. Those satellites all get their time from the U.S. Naval Observatory. The servers communicate with your iPhone via the Internet which then relays information to your Apple Watch. Corrections are done for time delay in communication.
Take a look at the video below...