Nostr Dev Team on Nostr: It depends on the client, maybe they are caching the relay list. There are many ...

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