See the documentaion about the [workflow states][1] for more info.
It is designed that if you take an Task a task from an UserTask also another user can do the same unless the task is finised.
It's now more like optimisic locking. Because with the task switch we had the problem that many users just clicked on a task and then not finished it, but for the time until the session of this user ended the task was not visible to anybody.
[1]: http://developer.axonivy.com/doc/latest/DesignerGuideHtml/ivy.concepts.html#ivy-wf-states