Why do you have achievements for the shivering isles on oblivion if you don't have the expansion pack?

Achievements for the Shivering Isles in Oblivion may appear if you are playing on a platform that allows for mods or if you have access to a version of the game that includes achievements tied to the expansion, even if you haven't purchased the expansion itself. Some versions of the game may enable achievements based on content that is technically part of the game but not actively accessed. Additionally, certain promotional events or updates can sometimes unlock achievements related to expansions regardless of ownership.