The tallest building in the world is not the tallest structure in the world

Because the tallest structure is not yet occupied.

While determining the world's tallest structure has generally been straightforward, the definition of the world's tallest building or the world's tallest tower is less clear. The disputes generally centre on what should be counted as a building or a tower, and what is being measured.
In terms of absolute height, the tallest structure is currently the Burj Dubai, although it does not currently hold the official title of "Tallest Building in the World" until its construction is completed and the building is officially opened. The current official holder of the "Tallest Building in the World" is held by Taipei 101.

Who knew it could be so complicated?