What's the difference between Oblivion GOTY and Oblivion GOTY Deluxe?

The Elder Scrolls IV: Oblivion Game of the Year Edition includes Knights of the Nine and the Shivering Isles expansions.

The Elder Scrolls IV: Oblivion Game of the Year Edition Deluxe includes Knights of the Nine and the Shivering Isles expansions plus Fighter's Stronghold Expansion, Spell Tome Treasures, Vile Lair, Mehrune's Razor and much more.

thats what it says on the store page

Oblivion GOTY contains the base game and Knights of the Nine and Shivering Isles expansion, while the GOTY Deluxe contains all that and more downloadable content.