On the first pass of the updater, the updater will download the latest copies at WoWInterface and install them, even if they are exactly the same. This is an unavoidable property of the system because there is no way for it to reliably determine what version you currently have installed. Some authors use different version numbers in their TOC file, which is the updater's primary source of information. Since we cannot rely on that file matching WoWInterface's data, it takes a step and installs everything. Once it has done that, it can then track version numbers itself and know reliably what is installed.

This will only happen once for each addon - immediately after registration. After that, the only time an addon will be updated is if a new version is posted or for some other reason the tracker file does not match WoWInterface's data (such as you reverted to a backup version).