Reported upstream: https://secure.phabricator.com/T6772
At, for example, https://phabricator.wikimedia.org/project/view/5/, we get a "quick" list of 10 open tasks in that project. The only suggestion that this might not be a complete list of all open tasks in that project is the tiny "View All" button in the top-right corner.
It needs to be made much clearer that the task list on the project page itself is incomplete. Presumably it is the 10 most recently active/most recently created tasks?