When you open an application in multiple tabs of the Internet explorer, the session object will be same for both tabs. That is why when you refresh User A's tab, it brings up the data of User B.
This behaviour was implemented in IE because, say for example the user is displayed with a list of items and he wants to view each one in different tabs, if the session is not shared then he has to login to each and every tab which will be a big pain for him.
Here is good http://geekswithblogs.net/ranganh/archive/2009/04/17/asp.net-session-state-shared-between-ie-tabs-and-ie8.aspx which does a deep dive into this topic.