Pretty sure realmid's are unique, and cases where they are not are connected realms where guid has id of host realm in guid. Battle.net api should still return real unique id of realm.
Code:
78-Altar of Storms, 78-Anetheron - US connected
1136-Bonechewer, 1136-Aegwynn - US connected
1129-Burning Legion, 1129-Archimonde - US connected (but not listed in official blog)
and so on.
Also UnitGUID is completely unreliable for NPC's:
I'm getting 1129 (Agamaggan-US, host for connection?) for "player" (self) on Archimonde-US, but NPC's have realmid 74, I also saw a player with realmid 74 (and he was from Black Dragonflight) that doesn't seems to connected to Archimonde. Also saw another player from Gul'Dan with realmid 74 in guid. May be human starter zone for Archimonde-US is hosted by Skullcrusher-US (74)? Because otherwise all of this doesn't make any sense.
Can't test what realmid battle.net API returns on that realm, because i'm on starter account where realid is disabled.