It depends on the client, maybe they are caching the relay list.

There are many sources for relays..  Hard coded (app) relays, nip-05 relays, extension relays, relays embedded in notes, nprofiles, and the "actual" kind 10002 list as defined in NIP-65

https://github.com/nostr-protocol/nips/blob/master/65.md

TLDR;  it's messy, implementations differ, and making it performant is a challenge