ownCloud team published the official pre-2.7 Linux packages for all distributions on software.opensuse.org, because they were built using the Suse OBS infrastructure. Downside was, too things can be controlled from ownCloud team in the OBS and on the Suse download mirrors.
Since 2.7, ownCloud team publishes official desktop Linux packages on download.owncloud.com
This is also what’s linked from the official ownCloud website The Desktop App for efficient syncing - ownCloud
Repos on ubuntu.com are community efforts without involvement from ownCloud team