I'm guessing this is because the used the OS settings and turned off Notifications on their device .
That makes the token unregistered. But then next time you request the token from OS, the OS gives you the same token.
I just asked a similar question about it with more detail. Take a look:
https://developer.apple.com/forums/thread/806522
Topic:
App & System Services
SubTopic:
Notifications
Tags: