This icon only appears if the update is not solely game content. Types consist of Game Content, Downloadable Content, Workshop Content, and Shader Pre-caching.

Previously the progress bar would only display the downloading content progress but not the disk allocation process which would make an update to appear completed when it was not. The downloads page is the most visible aspect of the update.

The update was restricted to Steam users who were opted into the client beta channel at the time, but it's now been rolled out to everyone, along with specific notes about what's been changed.